docs(guides): add pebbledb build guide under docs/guides

docs/guides/build_with_pebbledb.md

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+# Build with PebbleDB
+
+This guide explains how a Cosmos SDK chain gains PebbleDB support: when it is
+available natively, how to patch older chains that do not support it out of the
+box, and how to enable it at build time. It targets two audiences:
+
+- **Node operators** who want to understand what Dockermint does under the
+  hood when they select the `db_backend = "pebbledb"` flavour in a recipe.
+- **Dockermint contributors** working on the `builder` module or on new
+  recipes that need to expose PebbleDB as a flavour.
+
+## 1. Native support
+
+PebbleDB is supported natively starting with
+`github.com/cometbft/cometbft-db` **v0.10.0**.
+
+In practice, this corresponds to chains built on top of:
+
+- CometBFT **v0.38.0** or newer, and
+- Cosmos SDK **v0.50** or newer.
+
+Any chain whose `go.mod` already pins `cometbft-db >= v0.10.0` can be compiled
+with PebbleDB without any source-level patching: only the appropriate build
+tags and `ldflags` are required (see section 3).
+
+Any chain pinning an older version of `cometbft-db`, or still relying on the
+legacy `github.com/tendermint/tm-db`, does **not** support PebbleDB natively
+and must be patched before compilation (see section 2).
+
+## 2. Patching older chains
+
+For chains that predate `cometbft-db v0.10.0`, PebbleDB support is available
+through community-maintained forks published by `notional-labs`. These forks
+are drop-in replacements, injected via `go mod edit -replace`.
+
+Run the following commands from the chain's Go module root (the directory
+containing `go.mod`) **before** running `go build`:
+
+```bash
+# Always required for chains below cometbft-db v0.10.0
+go mod edit -replace \
+    github.com/cometbft/cometbft-db=github.com/notional-labs/cometbft-db@pebble
+
+# Required only if the chain still imports the legacy tm-db
+go mod edit -replace \
+    github.com/tendermint/tm-db=github.com/notional-labs/tm-db@v0.6.8-pebble
+
+# Required only if the chain imports github.com/cosmos/cosmos-db
+go mod edit -replace \
+    github.com/cosmos/cosmos-db=github.com/notional-labs/cosmos-db@pebble
+
+# Resolve the new dependency graph
+go mod tidy
+```
+
+Notes:
+
+- The three `replace` directives are independent. Apply only the ones that
+  match modules actually imported by the chain. A quick check:
+
+  ```bash
+  grep -E 'cometbft-db|tm-db|cosmos-db' go.mod
+  ```
+
+- The `@pebble` and `@v0.6.8-pebble` identifiers are the branches and tags
+  maintained by `notional-labs` that carry the PebbleDB backend.
+- If a specific revision is required (for reproducibility or to pick up a
+  fix), replace `@pebble` with an explicit commit hash or tag, for example
+  `@v0.11.0-pebble-1`.
+- Expect some trial and error. The `notional-labs` forks are community
+  maintained and ship several branches and tags that are not always mutually
+  compatible with every chain version. If `go mod tidy` or the build fails
+  with resolution or API errors, try another tag (for example swap `@pebble`
+  for `@v0.11.0-pebble-1`, `@v0.9.1-pebble`, or a specific commit hash) until
+  a combination compiles cleanly against the target chain.
+- All of these forks live under the `notional-labs` GitHub organization at
+  <https://github.com/notional-labs>. Browse the repositories directly to
+  discover the available branches and tags for each module.
+
+Once the `replace` directives are in place and `go mod tidy` has succeeded,
+the chain can be compiled exactly as a natively-supported chain.
+
+## 3. Enabling PebbleDB at build time
+
+PebbleDB is gated behind a build tag. It must be enabled explicitly by
+passing both a build tag and an `ldflag` to `go build`:
+
+```bash
+go build \
+    -tags 'ledger,pebbledb' \
+    -ldflags "-X github.com/cosmos/cosmos-sdk/types.DBBackend=pebbledb" \
+    -o /path/to/output/binary \
+    ./cmd/<chain-daemon>
+```
+
+What each flag does:
+
+- `-tags 'ledger,pebbledb'` compiles the PebbleDB code path into the
+  binary. The `ledger` tag is kept to preserve hardware wallet support; add
+  any other tags the chain normally requires.
+- `-ldflags "-X github.com/cosmos/cosmos-sdk/types.DBBackend=pebbledb"` sets
+  PebbleDB as the default backend baked into the binary, so a freshly
+  initialised node uses PebbleDB without any additional configuration.
+
+A more complete invocation, mirroring what is typically wanted for a release
+build, looks like:
+
+```bash
+go build \
+    -mod=readonly \
+    -trimpath \
+    -tags 'ledger,pebbledb' \
+    -ldflags "\
+        -X github.com/cosmos/cosmos-sdk/version.Name=<chain-name> \
+        -X github.com/cosmos/cosmos-sdk/version.AppName=<daemon-name> \
+        -X github.com/cosmos/cosmos-sdk/version.Version=<git-tag> \
+        -X github.com/cosmos/cosmos-sdk/version.Commit=<git-commit> \
+        -X 'github.com/cosmos/cosmos-sdk/version.BuildTags=ledger,pebbledb' \
+        -X github.com/cosmos/cosmos-sdk/types.DBBackend=pebbledb" \
+    -o /usr/local/bin/<daemon-name> \
+    ./cmd/<daemon-name>
+```
+
+After installing the resulting binary, confirm that PebbleDB is compiled in:
+
+```bash
+<daemon-name> version --long | grep -i build_tags
+```
+
+The output should contain `ledger,pebbledb`. Existing nodes that are
+switching from `goleveldb` must also update their `config.toml`
+(`db_backend = "pebbledb"`) and either re-sync, import a PebbleDB snapshot,
+or convert an existing `goleveldb` database with
+[Pebblify](https://github.com/Dockermint/pebblify), since the two backends
+are not on-disk compatible.
+
+## 4. Mapping to Dockermint
+
+All the steps above are performed by Dockermint's `builder` module whenever
+a recipe exposes `pebbledb` in its `[flavours.available]` section and the
+user selects it, either through CLI arguments, `config.toml`, or the recipe
+defaults:
+
+```toml
+[flavours.available]
+db_backend = ["goleveldb", "pebbledb"]
+
+[flavours.default]
+db_backend = "goleveldb"
+```
+
+At build time, Dockermint resolves the selected `db_backend` flavour,
+injects the required `go mod edit -replace` directives for chains that
+predate native support, and forwards the `pebbledb` build tag along with the
+`-X github.com/cosmos/cosmos-sdk/types.DBBackend=pebbledb` linker variable
+through the recipe's `[build.linker.flags]` and `[build.linker.variables]`
+tables.
+
+## Summary
+
+1. Native support starts at `cometbft-db v0.10.0`
+   (CometBFT v0.38 / Cosmos SDK v0.50).
+2. Older chains must inject the `notional-labs` forks through
+   `go mod edit -replace` followed by `go mod tidy`.
+3. Enabling the backend is done at build time by adding the `pebbledb` build
+   tag and setting `types.DBBackend=pebbledb` via `-ldflags`.
+4. In Dockermint, all of the above is driven by selecting the `pebbledb`
+   value of the `db_backend` flavour in a recipe.