Convert docs to use pandoc

[grapoza]

Pandoc may need:

• Custom CSS/HTML template; possibly a way to generate something like that? If not, we can always script it out.
  • https://pandoc.org/MANUAL.html#templates
  • https://github.com/jgm/pandoc-templates/blob/master/default.html5
• https://pandoc.org/demos.html
• Build docs as part of the build script (if it needs to be done in appveyor, see what if you need pandoc? krlmlr/r-appveyor#82)
• Add YAML metadata file (https://pandoc.org/MANUAL.html#extension-yaml_metadata_block)

For Github:

• Add an output directory inside docs
• Add .nojekyll in the output directory
• Add output content to .gitignore, except for .nojekyll
• Add an npm task to build the docs
• Update sources to work with pandoc
• Add npm task usage to appveyor.yaml, and have it merge master into gh-pages on releases after docs build
  • https://gist.github.com/chrisjacob/833223#gistcomment-90988
  • Clone repo
  • Check out gh-pages branch
  • Copy output into <version> folder of gh-pages branch
  • Commit
• Possibly have a default index.html that auto-redirects to the latest version?