Generate man pages with hidden 'man' command

[muesli]

Todo: We should auto generate them in a GitHub workflow that's triggered by every push to master.

[bdrung]

This should be in a separate commit.

[bdrung]

Reviewed the man pages from a users persective:

• obs-cli.1 should list all subcommand (not just reference them indirectly in "SEE ALSO")
• obs-cli-completion is not needed since that is not called directly by the user
• It would be nice if each man page for the subcommand would mention all (e.g. obs-cli-label.1 would document "label countdown" and "label text".) Otherwise you have to jump too much between the man pages.
• The required parameters are often not correctly documented (e.g. "obs-cli scene switch [flags]" where [flags] is not documented, but this command expects a scene name).
• The descriptions could be more than just one sentence per command.

Can you add a section to the README that explains how to build the man pages?

It would be nice if obs-cli could ship bash completion config files.