Build: Add `SPHINX_LINKCHECK` build option
Also,
- Fix found broken links
- Use HTTPS instead of HTTP for links reported as permanently moved by the links checker
- Allow
sphinx-build
to output messages to the stdout (w/o redirection to file) if the user configures the build with verbosity log level (it's nice to see the link checker messages at build time)
Backport: release:HEAD~3^2
Topic-rename: sphinx-linkcheck
Edited by Brad King