Render “About this documentation” on the docs landing page

[Han5991]

Fixes: #60667

• Copy the documentation.md output to index.html/.json so /docs/latest/api/ immediately shows the
  introduction instead of a duplicated TOC
• Harden tools/doc/allhtml.mjs and tools/doc/alljson.mjs against the new index structure by skipping
  anchor tags without href, replacing the TOC picker markup, and keeping anchor validation intact

[nodejs-github-bot]

Review requested:

• @nodejs/web-infra

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.01%. Comparing base (33f982e) to head (9cc3584).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #60673      +/-   ##
==========================================
- Coverage   88.53%   88.01%   -0.53%     
==========================================
  Files         703      703              
  Lines      208077   208077              
  Branches    40078    39995      -83     
==========================================
- Hits       184224   183131    -1093     
- Misses      15888    16923    +1035     
- Partials     7965     8023      +58     

see 105 files with indirect coverage changes

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[aduh95]

I'm not sure this is desirable, it current behavior is a much better UX for mobile users IMO

[Han5991]

I'm not sure this is desirable, it current behavior is a much better UX for mobile users IMO

I’d stick with the current layout. The duplicate TOC isn’t great on desktop, but on phones/tablets that list
is the only quick way to jump into the API docs. Without it, mobile users hit /api/ and have to open the picker
before they can navigate, which is a step backward

[aduh95]

COuldn't have said it better myself, I guess we can close this then