The installation docs we had were very outdated. They recommended
easy_install (which isn't ideal for modern packages), spoke of now
unsupported configurations, and referenced old version numbers. The
Windows docs in particular were unhelpful, basically saying there aren't
any options on Windows (rather than pointing to Bitnami).

This change cleans up the docs quite a bit. It removes a bunch of stuff
we don't want to mention in the Linux installation docs (such as the
outdated requirements for development installs). It switches everything
to pip, places some notes around PySVN vs subvertpy, and more. The end
result is a more streamlined, up-to-date document.

The Windows docs now point to Bitnami, instead of telling people they're
out of luck.

Built the docs and read through them.
Tested new links.