Rewritten "extend cms" docs (#1671)

Hopefully this commit can be reverted once we fix the
layout manager to work with all four directions (north, south, east, west).
A "bookmark bar" makes more sense as an example than having the links
in the menu, and it allows us to illustrate the CMS layout techniques.
FIX ConfigManifest regenerating every request if variantKeySpec is an empty array()
Use httpError() instead of non-existent HTTPResponse_Exception class
Add some docs about admin-side HTML sanitisation