Overhaul of documentation

Changes to content:
- More meaningful welcome on index.html, with
  contents table moved to a separate page

- Add page about how to install. It also incorporates
  details of the directory structure

- The python autodoc sections now renamed to
  "--- reference" to be a bit clearer

- Add diagrams illustrating the database structure,
  to go with the page about the models.

- Add the diagrams explaining regridding to the
  ingestion module reference page

- More content added to all the pages in general

Changes to appearance:
- Add the ARGANS logo (with link) to the bottom
  of the sidebar

- Force google fonts to use https link, otherwise
  they don't get displayed when pages are viewed
  over https

- Change the style of the headings. Now only the
  h1 headings use the Limeight font and are in a
  grey box. Other headings use Raleway font and
  are just larger red text, no box.

- Make the sidebar show the full contents tree for
  the whole site, not just the current page. Note
  that this requires an extension to be installed.

- Remove the redundant link to homepage on left
  side of relbar, and replace with a link to github.

- Square bullet points in the main text, to match
  the sidebar