Developer: Document Budgie Panel Widget Development

This is a pretty substantial task but the premise is to provide a clear step-by-step guide to developing a Budgie Panel widget. This should include:

Documentation on the APIs themselves
Documentation resources related to C and Vala
Clonable templates for folks to build on top of, with documentation on our settings UI system
Testing widgets, recommendations for backing up and restoring configuration

BuddiesOfBudgie / docs