UG: Massive role clean-up.

- Some roles were combined to reduce the amount of roles in general.
  Current roles are explained in roles.rst file.
- Took standard reST roles into use when appropriate (e.g. code).
- With custom rules, emulated Sphinx rules when applicable (e.g. file).
- Configured single backtics to be an alias for code (e.g. `example`).
- Use double backtics as standard alias for literal (e.g. ``example``).

The main motivation was simplify using roles in general to make it
easier to use intallation instructions both as INSTALL.rst (#1779) and
as part of UG. Using standard roles when possible as well as aliases
`foo` and ``bar`` ease that a lot.