feat: migrate knowledgebase nav to the docengine subsystem (#2795)

> **Draft — depends on coreweave/docengine#183.** Do not merge until
#183 is merged and this repo's `docengine` submodule is bumped to
include it (Dependabot bump). The workflow calls `docengine
knowledgebase apply`, which only exists once the pinned submodule
contains #183.

## Summary

Replaces the bundled `scripts/knowledgebase-nav/` toolchain with the
DocEngine `knowledgebase` subsystem. The Python that was byte-for-byte
duplicated with `coreweave/docs` now lives once in the engine and
reaches hosts via Dependabot.

## Changes

- **`docengine-site/knowledgebase.yaml`** — the old `config.yaml` minus
`mintlify_root` (the support root is resolved from `docengine.yaml`
`output_subpath` = `.`). Products and allowed keywords are unchanged.
- **`docengine-site/templates/`** — the two MDX templates moved here.
W&B keeps its own product-index template variant (`title: "Support: …"`,
no `sidebarTitle`); templates are per-host.
- **Removed `scripts/knowledgebase-nav/`** — script, report, tests,
README, `Architecture.md`, `requirements.txt`. The engine owns the logic
and tests now.
- **`.github/workflows/knowledgebase-nav.yml`** — slimmed to:
authenticate + init the `docengine` submodule (`DOCENGINE_TOKEN`), `pip
install -e ./docengine`, `docengine knowledgebase apply --report-file
…`, upsert the PR comment (same `<!-- knowledgebase-nav-report -->`
marker), and commit the support output back with the
`wandb-docs-pr-writer` App token. The engine reports its exact changed
files, so the old `git add --dry-run` candidate loop is gone.

## Behavior change (forks)

Generation now needs the **private** `coreweave/docengine` submodule,
which fork PRs cannot clone. The job runs for **same-repo PRs and manual
dispatch only**; fork PRs are regenerated by a maintainer after merge
(the former bundled script could generate but never push on forks).
Workflow/job names are unchanged so any required-check names still
match.

## Verification

The engine change (#183) was proven behavior-preserving by a
differential against this repo's real
`scripts/knowledgebase-nav/generate_tags.py` over the live `support/`
tree (this is the `output_subpath: "."` case): the generated trees are
byte-identical except the intended `template:` provenance line
(`scripts/knowledgebase-nav/templates/…` →
`docengine-site/templates/…`); `support.mdx` and all article footers are
byte-identical. Report markdown is byte-identical to the legacy
`pr_report.py`.

## Merge order

1. Merge coreweave/docengine#183.
2. Bump this repo's `docengine` submodule to include it (Dependabot).
3. Mark this PR ready and merge.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: wandb-docs-pr-writer[bot] <wandb-docs-pr-writer[bot]@users.noreply.github.com>

Author: 3 people

.github/workflows/knowledgebase-nav.yml

@@ -1,34 +1,23 @@
 # ---------------------------------------------------------------------------
-# Knowledgebase Nav Configuration
+# Knowledgebase navigation configuration
 # ---------------------------------------------------------------------------
-# This file defines the products and allowed keywords (tags) for the
-# knowledgebase nav generator. The generator reads this file to know which
-# products to crawl and which keywords are valid for each product.
+# Consumed by the DocEngine `knowledgebase` subsystem (`docengine knowledgebase
+# apply`). It defines the products and allowed keywords (tags) the subsystem
+# crawls under <output_subpath>/support/<slug>/articles/.
+#
+# There is no `mintlify_root` key: the support root is resolved from
+# docengine.yaml `output_subpath` (here, the repo root ".").
 #
 # To add a new product:
-#   1. Add a new entry under "products:" with slug, display_name, and
-#      allowed_keywords.
-#   2. Create the directory support/<slug>/articles/ in the repo.
+#   1. Add an entry under "products:" with slug, display_name, allowed_keywords.
+#   2. Create support/<slug>/articles/ under the output_subpath.
 #   3. Add at least one article with keywords from the allowed list.
 #
-# To add a new keyword to an existing product:
-#   1. Add the keyword string to that product's allowed_keywords list,
-#      in alphabetical order.
-#   2. Use the exact same string (case-sensitive) in article front matter.
-#
-# If an article uses a keyword not listed here, the generator still creates
-# the tag page but prints a warning. Resolve the warning by adding the
-# keyword to this file.
+# If an article uses a keyword not listed here, the subsystem still creates the
+# tag page but warns. Resolve the warning by adding the keyword here.
 # ---------------------------------------------------------------------------
 
-# Project-specific settings
-# -------------------------
-# mintlify_root: Path from the GitHub repository root to the Mintlify root
-#   (the directory containing support/ and support.mdx). Use "." when the
-#   Mintlify root is the GitHub repo root, or a subdirectory name (for
-#   example "public-docs") when the docs live in a subfolder.
 # badge_color: Mintlify <Badge> color used for tab and featured Badges.
-mintlify_root: "."
 badge_color: "orange"
 
 products: