Add "Skip to main content" link for keyboard navigation in rustdoc
## Summary
This PR adds a "Skip to main content" link for keyboard navigation in rustdoc, improving accessibility by allowing users to bypass the sidebar and navigate directly to the main content area.
## Changes
- **`src/librustdoc/html/templates/page.html`**: Added a skip link (`<a class="skip-main-content">`) immediately after the `<body>` tag that links to `#main-content`
- **`src/librustdoc/html/static/css/rustdoc.css`**: Added CSS styles for the skip link:
- Visually hidden by default (`position: absolute; top: -100%`)
- Becomes visible when focused via Tab key (`top: 0` on `:focus`)
- Styled consistently with rustdoc theme using existing CSS variables
- **`tests/rustdoc-gui/skip-navigation.goml`**: Added GUI test to verify the skip link functionality
## WCAG Compliance
This addresses **WCAG Success Criterion 2.4.1 (Level A)** - Bypass Blocks:
> A mechanism is available to bypass blocks of content that are repeated on multiple web pages.
## Demo
When pressing Tab on a rustdoc page, the first focusable element is now the "Skip to main content" link, allowing keyboard users to jump directly to the main content without tabbing through the entire sidebar.
## Future Improvements
Based on the discussion in rust-lang/rust#151420, additional skip links could be added between the page summary and module contents sections. This PR provides the foundation, and we can iterate on adding more skip links based on feedback.
Fixesrust-lang/rust#151420
r? @JayanAXHF
ab0689919e