* Improve homepage semantic HTML and accessibility for AI crawlers

- Add <main> landmark wrapper to homepage template
- Fix mismatched heading tags (<h3> closing with </h2>)
- Use <article> elements for feature cards instead of <div>
- Add aria-labelledby attributes to sections
- Add visually-hidden label for chat textarea (WCAG 1.3.1, 3.3.2, 4.1.2)
- Add autocomplete attribute to chat input
- Add .visually-hidden CSS class for accessibility
- Support optional home-ai-summary and home-image-alt attributes
- Wrap tertiary links in <nav> element

Addresses AI audit findings for docs.redpanda.com homepage including
semantic ratio, form accessibility, and content structure issues.

* Fix review findings: clip-path, aria-labelledby fallbacks, alt escaping

- Replace deprecated clip: rect() with clip-path: inset(50%) in .visually-hidden
- Add fallback h2 headings for aria-labelledby references when titles are absent
- Use double-stash escaping for alt attribute values to prevent injection

* add limited availability badge

* update

* fix

* remove unrelated changes

* adjust style

* fix lint and code rabbit errors