Style Guide

inforporate content from civicrm.org/developer-resources into "community" page

Conflicts:
	mkdocs.yml

api/usage.md - The OO example doesn't work. Fix it.

new page: Prerequisite skills

Move/codebase

merge from core

I hope it's okay for me to merge this without any reviewers. Seems small enough and useful/important. Please let me know if I'm overstepping. 

adding document-ready function to js

Scroll to active navigation menu item on page load, when necessary

Admonitions and other improvements to markdown documentation

Move/createmoduleextension

merge from core

Added links to documentation repositories.

and more tidying up and grammar etc