#659 GTK3V2MAIN: Doxygen main-page/dev-docs improvements, Phase D (vendor Doxygen Awesome theme)

[Allen D Byrne]

Fourth and final phase of the effort to make the generated Doxygen internals docs (docs/doxygen/) usable. Follows Phases A (SF #656), B (SF #657), C (SF #658). Full plan: git's .claude/doxygen-mainpage-plan.md (working notes, not committed). Branched from bug-658's tip (not independently) since this phase has a real functional dependency on bug-656/Phase A's GENERATE_TREEVIEW=YES setting -- must merge in order #656 -> #657 -> #658 -> this one.

Vendors Doxygen Awesome (https://github.com/jothepro/doxygen-awesome-css) via CMake FetchContent, pinned to the commit the v2.4.2 tag pointed to at fetch time (d52eafe3e9303399fda15661f3d7bb8fe3d7eabc) rather than the moving tag name. MIT licensed.

Theme variant and extensions were explicit user choices: sidebar-only layout (over the base top-nav variant), plus all four optional JS extensions -- dark mode toggle, fragment copy button, paragraph anchor links, interactive TOC.

New docs/doxygen/header.html: Doxygen's own default header (from doxygen -w html against the pinned Doxygen 1.15.0 binary) with just the 4 extensions' script tags added right before , nothing else hand-edited.

One real integration bug found and fixed by actually rendering the output, not just checking docs-doxygen's exit code: with DISABLE_INDEX=NO (the theme's own documented sidebar-only setting), Doxygen's classic top tab bar still renders and carries the search box at its tail end. The sidebar-only theme's CSS clips that whole tab bar out of view, taking the search box and dark-mode toggle button down with it -- a rendered screenshot showed neither visible at all. Fixed by setting DISABLE_INDEX=YES instead, which removes the now-redundant tab bar (GENERATE_TREEVIEW's sidebar already covers that navigation) and puts the search box directly in the un-clipped title row -- verified both features render and work via a follow-up screenshot, then confirmed interactively.

Verification: docs-doxygen exits 0 with FAIL_ON_WARNINGS active (CI-pinned Doxygen 1.15.0 binary). Zero local-filesystem-path leaks in the generated HTML. Full rebuild clean, ctest 10/10 pass, on both git and Hg trees. Git PR: https://github.com/adbyrne/XTrkCAD/pull/28 (GTK3V2MAIN).