docs: Ship the MkDocs documentation site with the installation

[petrasovaa]

Part of the transition to markdown-based documentation.

TL;DR:
make install/make bindist and cmake --install now build and ship the MkDocs documentation site, which g.manual and the GUI then use; if mkdocs is missing, the site is skipped with a warning and nothing else changes.

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Ship the MkDocs documentation site with the installation. g.manual and the GUI help viewer already prefer $GISBASE/docs/mkdocs/site` when it exists, so installs pick it up automatically; the legacy HTML pages remain as fallback until a later phase removes them.

The site is built at install time rather than in the default build, so mkdocs (man/mkdocs/requirements.txt) does not become a build dependency and CI is unaffected:

• Autotools: make install and make bindist build the site into the dist tree via a new top-level build-mkdocs target; the install copy then includes it.
• CMake: an install(CODE) step runs mkdocs build during cmake --install.

When mkdocs is not on PATH at install time (e.g. it is in a user environment but the install runs under sudo), a site built beforehand (make -C man build-mkdocs or the build-mkdocs CMake target) is still installed; without one, the site is skipped with a warning. A failed build likewise warns instead of aborting the install.

Also align the local targets' SITE_NAME with the site published by the documentation workflow: the title is now
GRASS <major>.<minor> Documentation everywhere, while the exact version (including dev) stays in the copyright footer.

Note for packagers: the site adds roughly 120-200 MB, temporarily double accounting next to the legacy HTML pages until those are removed.

Implemented with AI assistance (Claude Fable).