Add VarBC documentation theory and user guide

[xinzhang8noaa]

Description

Add VarBC theory and user guide - first version.

Issue(s) addressed

None

Dependencies

None

[shlyaeva]

From the old PR (https://github.com/JCSDA-internal/jedi-docs-old/pull/289) looks like @fabiolrdiniz approved, @dickdee commented that this documentation is ok for a start and @ytremolet requested changes that may have been addressed.
Should we merge this or does anyone have time to re-review?

[ytremolet]

Yes, let's merge and open another PR if needed.

[ytremolet]

But it does need another approval.

[mmiesch]

Uh oh - after merging this, my local sphinx build is failing with this:

14:16 $ make html
Running Sphinx v3.2.1

Extension error:
Could not import extension sphinxcontrib.bibtex (exception: No module named 'sphinxcontrib.bibtex')

@xinzhang8noaa - did you forget to include this in the committed files?

[mmiesch]

Never mind - I just installed it with conda and it worked

[xinzhang8noaa]

@ytremolet requested changes are addressed. If ther is any other issue, we can address another PR. Thank everyone !