Extract user documentation app to a separate service

[rod-glover]

The PDP user documentation is currently served by an app within the main PDP app. This app could become (semi)independent of the PDP app. Let's consider what that would take:

• Extract the doc app as a single WSGI app. In PDP, the app is already simple and could probably become even simpler given it would not need a dispatcher. It may turn out to be a trivial application of Flask serving static files.
• Extract the Sphinx documentation infrastructure to the new app. This includes:
  • Sphinx installation requirements
  • Sphinx configuration and framework from ./doc -- including index.rst, and likely the FAQ and other cross-portal documentation from PDP
  • doc_helpers module
  • Doc-specific code in PDP setup.py
• Add code to copy in the portal documentation from the main PDP repo, as well as from MDP and any other portals. PDP now also provides "external" portals. Everything is external to the doc app.
• Add in-repo deployment infrastructure (Docker image build action, etc.)
• Add IT deployment infrastructure (deployment directory, proxy config, etc.)

Verdict: Entirely doable, but will take some effort.