Better landing page

[dgoetz]

One feedback on my Pull request for the Github Profile was that docs.theforeman.org needs a better landing page if we want to direct users to it and I agree on this. While it is quite functional to have an overview over the supported and unsupported versions the current landing page does not guide a new user well.

So my ideas to improve this:

• A banner explaining the current state and linking to the old manual and plugin overview we want to replace here
• A short introduction to the project including an image
• Some explanation about the provided documentation and the used structure
• Better links for starting to read the documentation of the current version perhaps structured based on Foreman / Katello decision
• Shorter overview over supported / unsupported status linking to the version specific documentation

[maximiliankolb]

Relates to Making docs.theforeman.org the primary documentation source.

[ekohl]

This is exactly what I had in mind. I'll even take the blame for some of the current state since I wrote part of it. We can do better.