Complete documentation with example problems, FAQs, etc.

[bch0w]

The documentation page (https://seisflows.readthedocs.io/en/latest/index.html) is currently a direct port from the previous SeisFlows3 documentation, which was in a developmental state. Documentation pages which should be added to consider the next docs version "complete" include:

• Update documentation to remove old references of SeisFlows3 and SF3
• SPECFEM3D example problem and walkthrough
• How-To extend SeisFlows yourself (i.e., writing your own super class)
• Frequently Asked Questions (FAQ)
• Debugging SeisFlows

[bch0w]

See SPECFEM Users Workshop material (announced in #136) for a plethora of workshop material which explains a lot of the underlying structure of SeisFlows.

In particular the Day 3C notebook gives an introduction to SeisFlows.

[bch0w]

Documentation greatly improved in #146 and #147

[bch0w]

Closing in favor of new issue #198