linkcheck option to ignore certain protocols

Is your feature request related to a problem? Please describe.

Run:

sphinx-build -b linkcheck source/ _build/linkcheck

Produces an error for each man:, for example:

( commands: line   18) broken    man:bash(1) -

conf.py has:

manpages_url = "man:{path}"

Markdown example:

{manpage}`bash(1)` - GNU Bourne-Again SHell

HTML output:

man:bash(1)

Firefox browser has an application content type of man (x-scheme-handler/man), which is handled by an xdg desktop file to convert a local man page to html and display it in the browser.

Describe the solution you'd like

An option added to sphinx-build -b linkcheck source/ _build/linkcheck to ignore specified protocols, like manpage url.

Current options for the linkcheck builder.

Describe alternatives you've considered

An alternative is to pipe the output to grep and filter out man: errors.

Start with the linkcheck builder options and the configuration documentation linked in the issue, then trace how sphinx-build processes the manpages_url setting in conf.py. Add the requested protocol-ignore configuration and document it alongside the existing linkcheck options. Done means configured man: links no longer produce linkcheck errors while other protocols remain checked.