Close #86

by passing custom options to the pygments PHP lexer, now that this is
possbile with pymdown-extensions 2.0

Per request from Josh, this needs to be changed ASAP.
We will find a better name at some point soon.

remove hookref-old.md and move all content into relevant files. Add wordpress setup information for hooks.

- update overview
- add tips and warnings
- add section on hook naming
- replace long code blocks (that focus more on other components) with simpler blocks that show hook usage only
- replace validate with validateForm
 - remove complicated example for setting custom values
 - update tokenValues function to match newer version
 - remove examples of executing raw SQL

Closes #90

This is for people who are writing code.  It would be great if all of them
also wrote docs, but...  a new dev should learn a bit about dev before they
start contributing to dev-docs.  ;)

(I appreciate "Documentation Driven Design" but not "Doc first, ask questions later".)

The menu sets up divisions between "Core/Extension" (which are similar but
different -- two styles of packaging and collaboration) and "API/Hooks"
(which are similar but different -- two types of interfaces).

that URL hacking works

(I looked at the content in this file to make sure there's nothing in it
that can't be found elsewhere. All content already exists in the wiki
and thus is moved over as part of auto migration for hooks.)