ci: fix mgmt api docs generator (#44121)

The [`docs-mgmt-api-update`
workflow](https://github.com/supabase/supabase/blob/master/.github/workflows/docs-mgmt-api-update.yml#L21)
uses `sparse-checkout`, only pulling `apps/docs` and patches. After PR
#42987, the Makefile's `redocly` commands were changed to run via `pnpm
exec` `redocly` from `packages/generator`, but that directory was never
checked out, causing the can't `cd` error. Adding `packages/generator`
to `sparse-checkout` makes the existing Makefile dependency explicit in
the workflow.

.github/workflows/docs-mgmt-api-update.yml

@@ -21,6 +21,7 @@ jobs:
           sparse-checkout: |
             apps/docs
             patches
+            packages/generator
 
       - uses: pnpm/action-setup@41ff72655975bd51cab0327fa583b6e92b6d3061 # v4.2.0
         name: Install pnpm