Swap internal doc. refs to relative links.

This commit addresses issue #1571, which requests that all internal
Kubernetes links are swapped to relative ones to better facilitate
browsing of documentation on local forks, not to mention make the
documentation have less needless boilerplate.

I have already signed the Google Contributor License Agreement; and
as an employee of Google, Inc., this work is subsumed by its authorship
clauses.  The behavior of the relative links has been validated in Mou
as well as on GitHub's content browser.

The only thing that has not been updated are non-Markdown file links
to content.  If you request that to be done, I can re-issue this branch with
said changes.