Doc: Updated Document paths

This patch modifes the documentation
in the following ways:

* Contributing, glossary and maintainers
  files, are moved into the
  the documentation directory
* The table of contents in index.rst.in are
  updated accordingly
* Coding_guide, contributing, maintainers and
  readme documetns, are updated accordingly

Change-Id: I40ea47f7f154142f1f02f4492ad53c3533109bb0
Signed-off-by: Galanakis, Minos <minos.galanakis@arm.com>