fix: date sitemap entries from their pages

The sitemap gave every page the build date. A page now states its own date
and a mkdocs hook passes it on, or leaves lastmod out. The values added to
the site's code spans today are also escaped: a backtick in a profile or
file name could close the span and inject markup.

.github/workflows/deploy-site.yml

@@ -20,6 +20,7 @@ on:
       - "scripts/generate_site.py"
       - "scripts/hashing.py"
       - "scripts/largefiles.py"
+      - "scripts/mkdocs_hooks.py"
       - "scripts/nativemode.py"
       - "scripts/packextras.py"
       - "scripts/packpaths.py"