[docs] Cleanup docs for new contributors

[haworku]

We have started getting outside pull requests 🎉 . This is great and let's clean up things a bit.

• Separate the releasing guide from contributing guide.
• Improve contributing guide to more clearly highlight how external PRs will be merged (currently we have to replicate work to a new branch for CI runs, eventually we have automation for this).
• Double check accuracy, readability of contributing guide and PR template in general.

[jim]

We could move the checklist part of the PR template into a section (instead of a comment at the beginning) so that the list is visible in PRs by default. I think showing it at the beginning in a comment can be confusing- what are we expecting folks to do with it?