Modernize the guides on creating extensions.
Review Request #188 - Created Dec. 1, 2016 and updated
This fleshes out the various pages on creating extensions. There's now a new, fully-detailed walkthrough of the extension class and the attributes/methods that need to be used. The guides to the file/package layout and packaging have been updated to talk about Wheels, static media bundling, dependencies, etc. The configuration and static media pages go into more details on the options available, along with examples. The database models page now talks about writing evolution files. The resulting documentation is now a lot more clear, helping truly guide extension authors to getting started. It should be easier for us to continue to maintain and update as we go forward. Testing Done: Built the docs and went through them, checking for spelling errors (though it's possible I missed some), links, source code, etc. (Some links are currently broken, just because there are some new pages that aren't part of this change.) Reviewed at https://reviews.reviewboard.org/r/8523/