This fleshes out our documentation on configuring repositories to
include instructions on each hosting service we support. For most, this
is just basic instructions on the types of repositories supported, how
to link accounts, and how to configure the repository information for
the given service.

Some docs go into more detail on extra configuration and features. For
instance, the GitHub documentation talks about how to configure a custom
OAuth app and how to request/grant permission on organizations.

This also includes documentation on VisualStudio.com and GitHub
Enterprise. While included from Power Pack, having all repository
configuration docs in one place is ideal for those getting started. The
Power Pack documentation will be updated later to point to this.

Documentation for Kiln has been intentionally left out, as Fog Creek has
renamed Kiln and changed some aspects of it. We'll need to sort out our
hosting service support before useful docs can be written.

Built/viewed the docs and tested links.
Checked for spelling errors.
Tested each set of docs against the realities of the hosting service.