Improve website accessibility to address Lighthouse audit findings from #7019

[Shanwis]

What this PR does:
This PR addresses a focused subset of the Lighthouse accessibility recommendations from Issue #7019, improving the website's accessibility score from 81 to 100 without changing its appearance or functionality.

Direct Lighthouse Failure Fixes

• value.html — Added alt="{{ $value_title }}" to tag → fixes Images missing alt attributes
• feature.html — Added aria-label with title on auto-generated "Read more" links → fixes Links without discernible name and Identical links
• footer.html — Moved aria-label="{{ .name }}" from
• to on icon-only footer links → fixes Links without discernible name (footer)
• _styles_project.scss — Added text-decoration: underline to footer a, .td-cover-block a, .td-box a → fixes Links rely on color to be distinguishable

General Improvements (not in Lighthouse failures)

1. _index.html — Added rel="noopener noreferrer" to 3 external links (Playground, GitHub, releases)
2. navbar.html — Added rel="noopener noreferrer" to external nav links
3. navbar.html — Added aria-label="Main navigation" to <nav> element
4. navbar-version-selector.html — Fixed aria-labelledby value from "navbarDropdownMenuLink" to "navbarDropdown" to match the element's id
5. sidebar-tree.html — Fixed aria-controls value from "td-docs-nav" to "td-section-nav" to match data-target
6. footer.html — Changed wrapping <div> → <footer> (semantic HTML)
7. footer.html — Changed CNCF description <div> → <p> (semantic HTML)
8. value.html — Changed <div class="value-title h2"> → <h2 class="value-title"> (proper heading hierarchy)
9. value.html — Changed <p class="value-body"> → <div class="value-body"> (fixes invalid <ul> inside <p>)
10. _styles_project.scss — Changed .long-description color from $gray-500 to #c9d1e0
11. _styles_project.scss — Added .btn-secondary custom color override

Why we need it:
A Lighthouse audit identified several opportunities to improve the website's accessibility. This PR addresses the remaining automated accessibility failures (after #7020's SEO changes) by:

• Ensuring ARIA attributes correctly reference their target elements
• Providing accessible names for interactive elements navigated by screen readers
• Meeting WCAG 2.1 AA color contrast requirements (4.5:1 for normal text)
• Using semantic HTML elements (<h2>, <p>, <footer>, <nav>) for proper document structure and landmark navigation
• Adding meaningful alternative text for images
• Ensuring links are distinguishable from surrounding text without relying on color alone

Which issue(s) this PR fixes:
Fixes #7019

Does this PR introduce a user-facing change?:

Yes.

• How are users affected by this change:
  • Screen reader users benefit from correct ARIA references, proper landmark regions, and descriptive link names
  • Users with low vision benefit from improved color contrast ratios meeting WCAG 2.1 AA
  • Users who cannot distinguish colors benefit from underlined links that are visually distinct from surrounding text
  • The homepage images now have descriptive alternative text for assistive technologies
• Is this breaking change: No
• How to migrate (if breaking change): Not applicable

[github-actions]

👋 Hi @Shanwis, welcome to PipeCD and thanks for opening your first pull request!

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[yashisrani]

@Shanwis looks good to me