Implement RestructuredText Substitutions

[FroggyFlox]

RestructuredText allows for the definitions of "variables" that can then be re-used in all source files throughout the project. We should explore implementing this substitution feature to help with propagating updates or changes throughout our entire docs with a single line change.

Possible use for this could be:

• Rockstor name + version(s)
• base OS name + version
• often misspelled names such as Rock-Ons, etc...
• and more...

Note, Sphinx already include some default substitutions: https://www.sphinx-doc.org/en/master/usage/restructuredtext/roles.html#substitutions. These include release and version that are defined in conf.py.

To define additional ones: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-rst_prolog

rst_prolog = """
.. |rockstor_project| replace:: The Rockstor Project
"""