feat(docs) Dynamically E2E test all docs-owned content (#48320)

Closes DOCS-1203

## Problem

The docs E2E workflow only ever tested one hardcoded page: the Next.js
quickstart. All other docs content had no E2E coverage.

## Solution

This PR expands the initial scaffolding to generalize the Next.js
quickstart tests, page runs and checks local links, to all pages
affecting Docs content:


- Add `resolveDocsScope` (`e2e/docs/utils/resolve-docs-scope.ts`) to map
changed guide and troubleshooting `.mdx` files to their `/docs/...` page
paths, and to expand changed `_partials` to every page that includes
them (including transitively, through partials nested inside other
partials). Federated guide sections (`graphql`,
`database/extensions/wrappers`, `ai/python`, `deployment/terraform`,
`deployment/ci`) and reference docs stay out of scope, and resolution is
capped at 20 pages to keep runtime bounded.
- Replace the single `quickstarts.spec.ts` test with a generic
`docs-pages.spec.ts` that loads whatever pages are resolved, asserting
each renders with an `<h1>` and that its docs-owned links resolve.
- Add `run-e2e-docs.ts` so `pnpm e2e:docs` resolves scope locally (from
commits since `origin/master`, plus staged/unstaged changes) and skips
Playwright entirely when nothing in scope changed.
- Update `.github/workflows/docs-e2e.yml` to widen the trigger paths to
all guides/troubleshooting/partials, resolve scope in a dedicated step,
skip the rest of the job when scope is empty, and accept a `page_paths`
input for manual `workflow_dispatch` runs.
- Rewrite `e2e/docs/README.md` to document the new scoping behavior, the
override envs (`DOCS_E2E_PAGE_PATHS`, `DOCS_E2E_BASE_REF`), and how CI
uses the suite.
- `pnpm e2e:docs:all` is also added to run tests on every page locally.
Good for scoping issues but should not be included in CI.

## Manual testing

Walk through the following steps to verify this works:

- [x] `pnpm e2e:docs` from repo root resolves the expected pages for a
local guide edit and can run against local dev
**Note:** Challenges with testing on local in part because of the long
lag for first page load. Recommendation to use a hosted URL is added to
docs.
- [x] Editing a shared `_partials` file resolves to every page that
includes it (including through nested partials)
- [x] `pnpm e2e:docs` exits cleanly with no Playwright run when no
in-scope files changed
- [x] `git diff --name-only ... | pnpm -C e2e/docs resolve-docs-scope`
prints the expected page list for a sample diff
- [x] Workflow run on a PR that only touches `e2e/docs`/workflow files
skips the Playwright steps
- [x] Manual `workflow_dispatch` run with `page_paths` set tests only
those pages
- [x] Run `pnpm e2e:docs:all` to run the suite on all docs content,
which takes awhile

## Next steps

After this PR merges, we have the scaffolding to add more fun tests like
a11y 😁

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **New Features**
* Added scoped Docs E2E runs that target eligible doc pages based on
changes, plus manual page-targeted runs and an “all eligible pages”
mode.
* Introduced `DOCS_E2E_PAGE_PATHS` (and updated base ref/base URL
behavior) to control which pages are tested.
* **Bug Fixes**
* Automatically skips Playwright setup when no relevant pages are in
scope; Playwright reporting now uploads only on failure.
* **Documentation**
* Updated the Docs E2E README with new run/CI behavior, troubleshooting
notes, and commands to inspect the resolved page list.
* **Tests**
* Added a Docs-owned pages E2E suite; removed the Next.js quickstart E2E
spec.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

package.json

@@ -37,6 +37,7 @@
     "e2e": "pnpm --prefix e2e/studio run e2e",
     "e2e:ui": "pnpm --prefix e2e/studio run e2e:ui",
     "e2e:docs": "pnpm --prefix e2e/docs run e2e:docs",
+    "e2e:docs:all": "pnpm --prefix e2e/docs run e2e:docs:all",
     "e2e:docs:ui": "pnpm --prefix e2e/docs run e2e:ui",
     "e2e:docs:local-smoke": "pnpm --prefix e2e/docs run e2e:docs:local-smoke",
     "perf:kong": "ab -t 5 -c 20 -T application/json http://localhost:8000/",
@@ -73,13 +74,6 @@
     "pnpm": "11.13",
     "node": ">=22.13"
   },
-  "keywords": [
-    "postgres",
-    "firebase",
-    "storage",
-    "functions",
-    "database",
-    "auth"
-  ],
+  "keywords": ["postgres", "firebase", "storage", "functions", "database", "auth"],
   "packageManager": "pnpm@11.13.1"
 }