docs: add admin screen performance PR testing checklist

[faisalahammad]

Summary

Adds a performance PR testing checklist for admin screens to the contributing docs, so contributors have a copy-pasteable smoke process for the heavy SCF admin screens before opening a PR. The project has no dedicated performance harness, so the checklist walks through manual browser checks plus reuse of the existing Playwright specs as a regression baseline.

Fixes #498

Changes

docs/contributing/admin-performance-testing.md (new)

The checklist itself. Covers when to run it, prerequisites, the screens most likely to regress (Field Groups list, Field Group editor, UI Options Pages list/editor, runtime options page, Tools), browser DevTools checks with JS console and PHP debug.log separated, a Playwright smoke block reusing existing e2e specs, and an optional ad-hoc profiling section using the browser's built-in profiler. Three copy-pasteable sh blocks.

docs/contributing/index.md

Before:

3. **Testing**
   - Unit testing
   - Integration testing
   - Bug reporting
   - Feature validation

After:

3. **Testing**
   - Unit testing
   - Integration testing
   - Bug reporting
   - Feature validation
   - For PRs that change admin screen rendering, see [Performance Testing for Admin Screens](admin-performance-testing.md)

Why: Discoverability. Contributors land on the index guide first, so the checklist is one click from the entry point.

.github/pull_request_template.md

Before:

<!-- Thanks for contributing ... Contributing Guidelines link -->

After:

<!-- Thanks for contributing ... Contributing Guidelines link -->

<!-- For PRs touching admin screen rendering, see the performance testing checklist:
https://github.com/WordPress/secure-custom-fields/blob/trunk/docs/contributing/admin-performance-testing.md -->

Why: Surfaces the checklist at PR creation time, exactly when a contributor is deciding what to test. Kept as an HTML comment so it does not change the rendered PR body.

docs/bin/manifest.json

Before: no entry for the new doc.
After:

{ "contributing/admin-performance-testing": { "slug": "admin-performance-testing", "parent": "contributing", "markdown_source": "https://github.com/wordpress/secure-custom-fields/blob/trunk/docs/contributing/admin-performance-testing.md" } }

Why: The manifest is auto-generated and committed (see #385 for the same pattern with releases.md). Regenerated via php docs/bin/generate-manifest.php so the new doc is discovered.

Testing

Test 1: markdown lint passes on the new doc

1. Run npm run lint:md
   Result: no output, exit 0.

Test 2: internal links resolve

1. Open docs/contributing/index.md and click the new link.
2. Confirm docs/contributing/admin-performance-testing.md exists.
3. Open .github/pull_request_template.md and confirm the HTML-comment URL points to the new doc on trunk.
   Result: all three links resolve.

Test 3: copy-pasteable blocks

1. Open the new doc.
2. Copy the Prerequisites block into a shell.
   Result: the sh blocks are runnable as-is (no placeholders that break copy-paste).

Checklist

• Follows the documentation style guide in docs/contributing/documentation.md
• Docs-only change, no runtime impact, backward compatible
• No unrelated refactors (diff limited to the four listed files)

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