[ADD] support for canonical urls in technical doc

* canonical_root setting is the path to the root of the canonical sphinx doc,
  if not set no canonical link is generated, must end with "/"
* canonical_branch defines the canonical branch to which to redirect, defaults
  to master

also various side-fixes:

* disabled permalinks in sphinx instead of hiding them via CSS
* improved generation of github links, removed _app global and setting of
  linkcode_resolve in conf.py