Update much of the documentation for the Extensions Overview page.

Review Request #5382 — Created Feb. 1, 2014 and submitted — Latest diff uploaded

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master

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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.