docs: auto-generate doc/api/headers.md from Doxygen XML

Replaces the hand-maintained `doc/api/headers.md` with a generated one
sourced from each header's `\file` brief and the public symbols
extracted by Doxygen into XML.  The CI workflow regenerates it on every
push to main that touches the public headers.

New files
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* scripts/gen-headers-md.py — parses doc/doxygen/xml/*.xml, groups
  headers by directory (CGAL public / CGAL internals / Core), and
  writes a markdown table with header path, first-sentence brief, and
  the public symbols declared at file scope.  Skips `detail::`
  namespaces and template-specialisation duplicates.
* scripts/regen-docs.sh — convenience wrapper:
  doxygen → gen-headers-md.py → coverage report.

Workflow changes
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.gitea/workflows/doxygen-pages.yml now:
  1. Runs `bash scripts/doxygen-coverage.sh` as an informational step
     (no fail threshold yet — the script supports `--threshold N` for
     when we're ready).
  2. Re-runs `python3 scripts/gen-headers-md.py` and warns if the
     file drifted from what's in main (operator should run
     `regen-docs.sh` locally before pushing).

Doxyfile hygiene
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`HTML_TIMESTAMP` was removed in Doxygen 1.10 → replaced with the new
`TIMESTAMP = NO` to silence the obsolete-tag warning.

Effect on the reviewer-facing landing pages
───────────────────────────────────────────
Every improvement to a `\file` brief at the top of a public header now
flows automatically into both:
  * the Doxygen HTML at https://tmoussa.codeberg.page/ConformalLabpp/
  * the markdown landing at doc/api/headers.md (rendered by Codeberg
    in the repo view)
…so writers have a single source of truth (the C++ source) and
readers see the same words in both surfaces.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Doxyfile

@@ -104,7 +104,8 @@ HTML_COLORSTYLE        = LIGHT
 HTML_COLORSTYLE_HUE    = 220
 HTML_COLORSTYLE_SAT    = 100
 HTML_COLORSTYLE_GAMMA  = 80
-HTML_TIMESTAMP         = NO
+# HTML_TIMESTAMP was removed in Doxygen 1.10; use TIMESTAMP=NO instead.
+TIMESTAMP              = NO
 HTML_DYNAMIC_SECTIONS  = YES
 GENERATE_TREEVIEW      = YES
 DISABLE_INDEX          = NO