Document development and release processes

[pouretrebelle]

There are lots of aspects of working in Primer ViewComponents that aren't well documented, and some of the content that is written down isn't very discoverable. I've found this to be a huge stumbling block in my first few weeks in the team - having to get a lot of context from asking questions in sync calls and not being able to make a lot of progress independently.

The areas I think need to be documented most are:

• Deprecation and removal process
• The different files that each component relies on (tests, stories, docs etc.)
• Release process
• Updating gh/gh
• Making changes that rely on Primer CSS changes

I've started this process by mapping out all the existing documentation for PVC, and also for PRC and PCSS to figure out what's best to have documented on primer.style and what's better as in-repo markdown files. Follow my audit and structure proposal in this FigJam file.

[pouretrebelle]

Closing for https://github.com/github/primer/issues/534 - noob didn't know the right place for this issue