Documentation-builder: rework to be more agnostic

[wdpypere]

• allows exports of variable annotations
• a lot of the assumptions originally made for configuration-modules-core are taken out and made configurable

[jrha]

I've read through the code (as much as I could) and tested this on a development system, a few observations:

1. Not all of the run-time dependencies are obviously documented, on CentOS 7 I had to:

   cpan install Pod::Markdown
   cpan install Pod::POM::View::Restructured
   pip install -U Sphinx
   yum install htop
   yum install mkdocs
   yum install python2-devel
   yum install python2-mock
   yum install python2-pip
   yum install rubygem-bundler

2. The README doesn't seem to match the options available with --help anymore?

3. The build script is hard-coded to call firefox which is a fun surprise if it is not available, I symlinked this to echo on my headless build systems.

4. I couldn't get it to work with Python 3, I didn't spend too long on it, but it looks like it is related to something in the vsc libraries rather than this script.

[jrha]

But on the plus side, it generated beautiful documentation!

[jrha]

I will use this to generate the 19.12 docs and may well merge after that.

[wdpypere]

@jrha thanks for picking this up!