Add deprecation guideline to development documentation

What does this MR do?

Adds some very basic information about our deprecation/removal process.

Context

This past milestone we had a number of questions in our team about how and when to deprecate and remove features. We were unable to find any relevant information in our Contributor and Development documentation, so we added this very basic page to start a discussion.

Disclaimer

I haven't personally been involved in many deprecation/feature removals, so it's very possible this content will need to be rewritten.

Also, if we have this info elsewhere in our documentation, I'm happy to close this MR!