Support nav links to a section of a page

[shenxianpeng]

A nav entry that links to a section of a page is reported as "not found in the documentation files", which fails strict builds:

nav:
  - About: about.md
  - License: about.md#license    # warned, and links to the .md file (404 on the built site)
  - License: about/#license      # works on the built site, but still warned

Fix:

• When the part before # is a documentation page, the link now points at the page's URL plus the anchor: about/#license, or about.html#license with use_directory_urls: false.
• Links whose path is the URL of a documentation page (about/#license) are logged at DEBUG level instead of being reported as not found. With use_directory_urls: false that URL doesn't exist, so it is still reported.
• References to missing pages (missing.md#x) are still reported, and absolute paths keep following validation.nav.absolute_links.

Verified end to end with and without directory URLs: the generated hrefs are correct from the home page, from nested pages, and for anchors on the current page. The anchor itself isn't validated against the target page; that would have to happen after rendering.

The nav section of the configuration docs now shows how to link to a section of a page.

Related Issue

No issue in this repository.

Checklist

• New tests added for new behavior (if applicable)
• Documentation updated (if applicable)
• Release notes docs/about/release-notes.md updated (if applicable)

🤖 Generated with Claude Code

[codspeed]

Merging this PR will not alter performance

✅ 28 untouched benchmarks

---

_{Comparing fix/nav-anchor-links (c4583a2) with main (6657fa0)}