Add version subcommand

[felixfontein]

Now that we have access to it, let's add the most obvious subcommand :)

[gotmax23]

We already have a --version option provided by antsibull_core.args.get_toplevel_parser(). It uses the version from the dist-info metadata instead of antsibull_docs.__version__, though.

[felixfontein]

Meh, I completely forgot about that one...

I guess I should work on a way to pass the real version into that one instead. Right now antsibull-docs --version for me shows 2.0.0a2, which was the current version when I last ran pip install -e...

[felixfontein]

I created ansible-community/antsibull-core#96 to be able to fix this.