doc/index.rst: add link to Dev Guide basic workfl.

doc/index.rst: add link to Dev Guide basic workfl.

This PR adds a link to the "Basic Workflow" section of the
Developer Guide on the landing page of docs.ceph.com.

This PR is meant to improve the documentation for developers
new to Ceph and to guide them to instructions that will allow
them to become full-fledged contributors to the Ceph project
as quickly as possible.

The "Basic Workflow" page of the Developer Guide contains
information that answers almost all of the questions that I had
about contributing to the Ceph project when I was new to it,
and I am finally acting on my long-held conviction that the
"Basic Workflow" page of the Developer Guide should have a more
prominent position in the documentation suite than it has had.

Signed-off-by: Zac Dover <zac.dover@gmail.com>
(cherry picked from commit c624a2a8ef71035eef34ee866ef7668f83a139e7)