Consolidate and update "Contributing" docs (in progress)

[apinnick]

We have a lot of files and links for contributing to this repository:

• foreman-documentation/readme
• foreman-documentation/CONTRIBUTING.md
• foreman-documentation/guides/README.md

Suggestions for improvement:

• Consolidate files.
• Remove obsolete information, duplicated info, and broken links.
• Edit guidelines so that they are not too overwhelming.
• Add link to RHSSG.
• Perhaps recommend installing Vale locally and running it on PR. The Vale linter in GitHub only looks for "errors". Most of the ISG and RHSSG guidelines are "warnings" or "suggestions".

[aneta-petrova]

Assigning to myself because I have an idea for how to address this and can work on it while also fixing #3219, which is related.