Split up the extension hooks docs, and modernize them.
Now that we have a lot more extension hooks, it's time to split up the

docs. The existing page was getting too big and hard to maintain,

especially as we go into more details on hooks.
Each hook now has its own page (with the exception of the ActionHooks,

which have one page for all three hooks, which are all basically the

same).
Some of the content was tweaked just a bit. Some formatting errors were

corrected. The extension examples were fleshed out to include imports

and the new initialize() method. The Template Hook docs now have the

rest of the template hook names. Small things like that. Otherwise, the

content is basically the same.

Viewed and checked every page for errors.