Track Lighthouse performance and accessibility improvements

## Summary

Track repository-owned improvements identified by a desktop Lighthouse audit of https://www.elastic.co/docs/release-notes/cloud-serverless on 2026-07-21.

## Baseline

| Category | Score |
|---|---:|
| Performance | 61 |
| Accessibility | 92 |
| Best Practices | 96 |
| SEO | 100 |

Key measurements:

- First Contentful Paint: 0.5 s
- Largest Contentful Paint: 1.0 s
- Total Blocking Time: 962 ms
- Speed Index: 4.7 s
- Cumulative Layout Shift: 0.004
- Transferred payload: 4.39 MiB
- Estimated unused JavaScript: 1.67 MiB

FCP, LCP, and CLS are healthy. The main opportunities are reducing post-paint JavaScript work and transferred code, correcting the pages dropdown semantics, and removing the failed API availability probe.

## Scope

Child issues cover independently implementable changes owned by `elastic/docs-builder`. Verify changes with a production frontend build and a clean/incognito Lighthouse run.

The supplied report warned that browser cache clearing timed out and IndexedDB data was present. It also attributed about 313 KiB of unused JavaScript to a browser extension, so exact scores and byte totals must not be treated as deterministic.

## Upstream findings

The following findings are not implemented by this repository and should be raised with the website/global-navigation owner separately:

- The global navigation `

` contains direct `` children for Pricing and Docs.
- `elastic-nav.js` causes about 219 ms of forced reflow and has no production source map.
- `MierB-Demi.woff2` lacks an effective `font-display` policy.
- Unused/minifiable website-search and Google Tag Manager assets are externally built.

## Completion criteria

- All repository-owned child issues are resolved or explicitly declined.
- Frontend unit, type, lint, and production build checks pass.
- Lighthouse is rerun in a clean browser profile and before/after results are recorded.