doc: add doc contributing guide, plus cleanup

Add documentation contributing information with project specific
recommentations along with some common reST and Sphinx constructs.
Reorganize contributing section to fit this new doc in.

Also

* Cleanup stray file (CODEOWNERS) from when docs were in their own repo.
* Remove licensing footer on generated pages (not really needed).
* Pull replacement strings into substitutions.txt for easier management.

Signed-off-by: David B. Kinder <david.b.kinder@intel.com>

doc/_templates/footer.html

@@ -1,8 +0,0 @@
-{% extends "!footer.html" %}
-{% block extrafooter %}
-    <p class="extrafooter">Documentation licensed under <a rel="license"
-    href="http://creativecommons.org/licenses/by/4.0/">(CC BY 4.0) <img
-    alt="Creative Commons License" style="border-width:0"
-    src="https://i.creativecommons.org/l/by/4.0/80x15.png" /></a></p>
-    {{ super() }}
-{% endblock %}