feat: alternate search index for nimbus (#38662)

* feat: alternate search index for nimbus

Create an alternate search index for Nimbus that filters out
feature-flagged pages (equivalent to setting all feature flags to
false).

Notes:
- Creates two new DB tables, `page_nimbus` and `page_section_nimbus`,
  which are filtered versions of `page` and `page_section`
- Makes `nimbus` versions of all the DB search functions
- Refactored the embedding upload script. Changes to make it faster (got
  annoyed by how slow it was when testing...), incorporate retries, and
  produce better summary logs.
- Upload script, when run with the environment variable
  ENABLED_FEATURES_OVERRIDE_DISABLE_ALL, produces and uploads the
  alternate search index
- Changed all the search calls in frontend/API to check for
  `isFeatureEnabled('search:fullIndex')` to determine whether to search
  the full or alternate index

* ci: produce nimbus search indexes on merge

* fix: turn full search index on

.github/workflows/search.yml

@@ -68,8 +68,12 @@ jobs:
         run: |
           pnpm run codegen:examples
           pnpm run embeddings
+          pnpm run embeddings:nimbus
 
       - name: Refresh embeddings
         working-directory: ./apps/docs
         if: ${{ inputs.refresh }}
-        run: pnpm run embeddings:refresh
+        run: |
+          pnpm run codegen:examples
+          pnpm run embeddings:refresh
+          pnpm run embeddings:nimbus:refresh