This fleshes out some of the TemplateHook docs, going into more detail

on a few hook points, along with linking to other extension pages that

are relevant for them. It also moves from a table-based display of names

to a definition-based display, which helps keep longer descriptions

readable.
The example has been updated to make use of alternate template hook

points, in order to avoid recommending its usage for custom CSS or

JavaScript (which static media bundles are a better solution for).
This is in preparation for some new updates to the list of available

template hook points that will be coming soon.

Built the docs and looked through them, checking for formatting and

linking errors.