doc: add doc writing guides w/common usages

I've collected some of the common issues encountered with doc reviews
into a new contributing document, and included use of the
Zephyr-specific extension for generating code building examples.

Updated conf.py and created an external list of substitutions making it
easier to manage them without editing the sphinx conf file (and
documented this).

Tweaked the comments in the application.py extension python code to
render better in the generated doc that extracts these comments (keeps
the documentation in the python code too to ease maintenance when
updates are made).

Updated the sample template to mention use of this sphinx extension.

fixes: #6831
fixes: #6811

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