Update severely outdated installation documentation.

- Simplify "Simple Installation" section
- Update "Release Verification" section to actually verify a tuf
  release and with a key of an active maintainer
- Update and simplify section about non-python dependencies
  (just point to installation instructions for underlying crypto
  backends, they are up-to-date and have become a lot easier)
- Add "Development installation" section

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Replace custom installation section in contribution docs with
pointer to updated installation documentation.

Also configure sphinx autosectionlabel for cross-document refs.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Make contributing document header sentence case for consistency
with other docs and shorten menu name in side navbar to stand out
less.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

Move release signature verification instructions to bottom of
install docs. The doc is short, so the section is still prominent
enough for promoting verification, but does not break the reading
flow as much anymore.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>

This is an ugly hack to also resolve the link when the document is
rendered in GitHub, where it is likely to be browsed, because it is
the community standard location for a GitHub repo's contributing
docs.

Coordinate with theupdateframework#1849 to better separate RTD docs with GitHub docs
in the future.

Signed-off-by: Lukas Puehringer <lukas.puehringer@nyu.edu>