#780 Remove hardcoded personal GitHub fork references from source (GTK3V2MAIN)

[Allen D Byrne]

Several checked-in files under GTK3V2MAIN hardcoded 'adbyrne' (the current maintainer's
personal GitHub fork identity) as if it were the project's canonical location, even though
SourceForge Mercurial is this project's authoritative source, not GitHub:

• docs/doxygen/DoxygenLayout.xml: the dev guide's "User Guide" nav tab linked directly to
  https://adbyrne.github.io/XTrkCAD/userguide/
• .github/workflows/release.yml: several published-site and release URLs hardcoded the same
  domain/account (deploy-pages index page, rolling latest-gtk3v2main release notes)
• distribution/win32/README-INSTALL.txt and distribution/osx/README-INSTALL.txt: shipped a
  SUPPORT line pointing at https://github.com/adbyrne/XTrkCAD
• distribution/flatpak/xtrkcad.manifest.yaml: example source tarball URL hardcoded the same
  account
• .github/README.md and README.md (git-only, not in Hg): similar hardcoded links/badges

Fix: DoxygenLayout.xml is now a CMake-configured template defaulting to a relative link (works
for any local build), with release.yml's package-docs job overriding it with the real deployed
URL computed from GitHub Actions' own repository context (github.repository_owner,
github.event.repository.name) instead of a literal account name. release.yml's other links use
the same context-expression approach. README-INSTALL.txt files are now .in templates defaulting
to the canonical SourceForge project page. The flatpak manifest (which needs one concrete
fetchable URL, can't be relative/variable) got an explicit placeholder instead of a real
personal account. README.md's CI badges now use repo-relative paths, which GitHub resolves
against whichever fork is hosting the page.

Verified: build-gtk3v2main's docs-doxygen and help-html targets both build clean with
WARN_AS_ERROR, and the generated dev-guide HTML's User Guide link correctly resolves to the
sibling app/doc/html build.

[strobelight]

I now get "your file couldn't be accessed" when clicking on the UserGuide link from dev docs:

file:///home/kshaffer/src/projects/XtrackCad-source/xtrkcad-fork-xtrkcad/build/docs/app/doc/html/

User guide at

/home/kshaffer/src/projects/XtrackCad-source/xtrkcad-fork-xtrkcad/build/app/doc/html/index.html

Generated dev guide at

file:///home/kshaffer/src/projects/XtrackCad-source/xtrkcad-fork-xtrkcad/build/docs/doxygen/html/index.html

Is there another make target to move it?

Also, can the link be to index.html instead of the directory?