fix: point old-version banner to the matching page on latest

[neon60]

Motivation

The "old version" announcement banner linked to a fixed docs root (https://rocm.docs.amd.com/en/latest/), so clicking it dropped the reader at the top of the latest documentation instead of the latest version of the page they were reading. From, say, a HIP 6.2.2 reference page, the banner sent readers to the docs home rather than the same reference page on latest.

This PR makes the banner link resolve to the matching page on the latest version, preserving the project and page path, while keeping a sensible no-JavaScript fallback.

Technical Details

• src/rocm_docs/rocm_docs_theme/static/bannerLatestLink.js (new) — a client-side script that, on load, finds links carrying the data-rocm-banner-latest-link attribute and rewrites their href from .../en// to the same under latest, preserving the project prefix and URL hash. If the current URL is not versioned (.../en//...), the link is left untouched.
  Missing pages fall back to Read the Docs' own 404 handling for the latest version.
• src/rocm_docs/theme.py — the three built-in banner variants (old release, release candidate, development) now tag their link with data-rocm-banner-latest-link. The link keeps its fixed .../en/latest/ href as the no-JS fallback; the script upgrades it at runtime. Registers the new script via add_js_file.
• Opt-in, decoupled from styling — the rewrite is triggered by the data- attribute, not by id="rocm-banner" (which remains the CSS styling hook only). This means a project that overrides announcement with its own link is left untouched unless it opts in by adding the attribute — so custom banner links are never silently hijacked.
• docs/user_guide/banner.md + docs/sphinx/_toc.yml.in — new user-guide page documenting the automatic banner, the matching-page behavior, and how to author a custom announcement (including opting a custom link into the rewrite). Added to the user-guide TOC.
• .wordlist.txt — adds focusable and href for the spellcheck gate.

Test Plan

• Build the docs with the banner forced on (an old-release build via ROCM_DOCS_REMOTE_DETAILS) and confirm the banner renders, ships the fixed-href fallback, and loads bannerLatestLink.js.
• Verify the URL-rewrite logic against representative URLs: a project page (.../projects/HIP/en/docs-6.2.2/reference/x.html), a hash fragment, an Instinct-subdomain page, and a non-versioned URL (must be left unchanged).
• Confirm a custom announcement link without the attribute is not rewritten.
• Pass the markdownlint and spellcheck CI gates on the new Markdown.

Test Result

• Old-release build: banner renders ("This is not the latest version…"), the baked href is the .../en/latest/ fallback, and bannerLatestLink.js is present on the page.
• Rewrite logic verified: .../HIP/en/docs-6.2.2/reference/cpp_language_extensions.html → .../HIP/en/latest/reference/cpp_language_extensions.html; project prefix and #hash preserved; non-versioned URLs return unchanged (left as-is).
• Override safety confirmed: a link with only id="rocm-banner" (no data- attribute) is not selected, so custom announcements keep their authored target.
• Spellcheck passes after adding focusable/href to the wordlist.

Submission Checklist

• Look over the contributing guidelines at https://github.com/ROCm/ROCm/blob/develop/CONTRIBUTING.md#pull-requests.

[neon60]

Test PR: ROCm/rocm-systems#10878