Document Admin docs sync workflow (#4497)

### Background

Admin UI extensions docs are generated from `Shopify/ui-extensions` and copied into `shopify-dev`. There is a required follow-up sync from `Shopify/extensibility` that needs to run last so App Home docs keep merged App Bridge and Polaris component data.

### Solution

- Add `packages/ui-extensions/docs/surfaces/admin/README.md` documenting the safe sync order.
- Call out that `docs:sync` must be rerun whenever `docs:admin` is rerun afterward.
- Include a copy-paste-safe `2026-04` command sequence.

### 🎩

Docs-only change. Self-reviewed the command order and verified shell commands are each on a single copy-paste-safe line.

### Checklist

- [x] I have updated relevant documentation

Author: MitchLillie

packages/ui-extensions/docs/surfaces/admin/README.md

@@ -0,0 +1,35 @@
+# Admin UI extensions docs sync workflow
+
+Admin UI extensions API docs are generated from this repository, usually from the quarterly API branch for the version being published, such as `2026-04`. The generated Admin docs are copied into `shopify-dev` under `db/data/docs/templated_apis/admin_extensions/<version>`.
+
+## Safe order of operations
+
+Run the App Bridge docs sync from `Shopify/extensibility` last. This is important because `yarn docs:admin` also writes the App Home docs data at `db/data/docs/templated_apis/app_home/generated_docs_data_v2.json`. The Extensibility sync merges and preserves the App Bridge and Polaris component docs for App Home in that file. If `yarn docs:admin` runs after Extensibility `docs:sync`, it can overwrite the merged App Home data and remove those App Bridge and Polaris component docs.
+
+Use this order:
+
+1. Generate Admin UI extensions docs from `Shopify/ui-extensions` for the target API version.
+2. Copy or promote the generated Admin docs into `shopify-dev` as needed.
+3. From `Shopify/extensibility`, run `pnpm --dir libraries/javascript/app-bridge docs:sync <version>`.
+4. If Admin docs generation is rerun afterward for the same version, rerun the Extensibility `docs:sync` command afterward too.
+
+## Example for `2026-04`
+
+Run Admin docs generation first:
+
+```bash
+cd /Users/mitch/src/github.com/Shopify/ui-extensions
+git checkout 2026-04
+yarn workspace @shopify/ui-extensions docs:admin 2026-04 2026-04
+```
+
+Copy or promote the generated Admin docs in `shopify-dev` as needed. The final Admin docs should live under `db/data/docs/templated_apis/admin_extensions/2026-04`.
+
+Then run the Extensibility App Bridge docs sync last:
+
+```bash
+cd /Users/mitch/src/github.com/Shopify/extensibility
+pnpm --dir libraries/javascript/app-bridge docs:sync 2026-04
+```
+
+If you run `yarn workspace @shopify/ui-extensions docs:admin 2026-04 2026-04` again later, run `pnpm --dir libraries/javascript/app-bridge docs:sync 2026-04` again afterward so `app_home/generated_docs_data_v2.json` keeps the merged App Bridge and Polaris component docs.