import static documentation from old gh-pages site; refresh some content

[edbennett]

To test:

• Create a Python environment with pymdown-extensions, mkdocs, and mkdocs-literate-nav
• Build the documentation: python -m mkdocs build
• Serve the site on a local HTTP server: cd site && python -m http.server 8003
• Go to http://localhost:8003 in your browser

[gray95]

Tested according to the instructions and everything works fine. Cursory glance at the generated docs - nothing glaring jumped out at me. mkdocs emits one warning, I don't know if it means anything.

$ python -m mkdocs build
INFO    -  Cleaning site directory
INFO    -  Building documentation to directory: /home/gsr/work/sa2c/Grid/site
WARNING -  Doc file 'api-description/introduction.md' contains an unrecognized relative link '../../dev', it was
           left as is. Did you mean '../dev/index.md'?
INFO    -  Documentation built in 0.13 seconds

[edbennett]

Yes, that warning is expected, due to the way that the Doxygen content is included. Thanks!