Update much of the documentation for the Extensions Overview page.
This rewrites much of the docs to simplify some of the language, clarify

some points, to modernize how things work, and to link to more useful

Django docs.
The general structure is still left the same. My next change is going to

split up the page into more dedicated topic pages.

Generated the docs, read through it, and tested links.
I did add a link to a js-extensions page, which doesn't yet exist. It's a

placeholder for docs I'm writing. This "broken link" is intentional.