Consistent version in documentation URLs
This now uses the plugin manual helper to redirect users. It consistently uses the same version as the REX documentation buttons.
Ideally it would also use the plugin_manual type in documentation_url, but that doesn't work in Foreman itself.
I am a bit worried that it now breaks all links, since https://theforeman.org/plugins/foreman_remote_execution/ only has versions 1.7, 1.3, and some 0.x versions. Given we're already up to 9.1 this isn't going to work for anything. Because of that I think it may be safe to assume nobody clicks the documentation buttons.
Does this replace #818 ?
I think it does. Note that the current documentation versions don't exist. https://theforeman.org/plugins/foreman_remote_execution/ only has very limited numbers. You can also choose to only document major versions in which case you can change rex_doc_version.
And we may need to reflect the change in Satellite's documentation link rewriting. I didn't check yet.
Trivial rebase done.
Closing in favor of https://github.com/theforeman/foreman_remote_execution/pull/820.