Add in some documentation on PHPIDS

Clarify that PHPIDS is not just about inputs which are saved to the
database, but actually all inputs (e.g. GET params) even ones that
aren't saved to the DB.

Migrate Token Documentation

ref:
https://docs.civicrm.org/dev/en/latest/documentation/style-guide/#describing-the-civicrm-user-interface

Update more wiki urls

Remove references to legacy extension types

Fix Wiki URLS to point to correct docs url

ref:
https://github.com/civicrm/civicrm-dev-docs/pull/301#issuecomment-324795431

Import "Verify a bug fix" wiki page

https://wiki.civicrm.org/confluence/display/CRMDOC/Verify+a+bug+fix

Clean up mkdocs.yml

Migrate CiviMail Reference Document

(They don't show up as sub-lists unless you use 4)

Originally there were different types of extensions - payment processors, reports & modules. For a long time only modules have been recommended and I feel documenting the others only adds confusion.

Migrate payment processor docs from wiki