Improve docs

[addisonElliott]

Changes

• Make a custom homepage for the docs that isn't the README
  • Including the README wasn't working so only the TOC was shown
  • Plus it's a little weird that the README references going to the docs when you're already there.
• Split content up into various sections to make navigation easier
  • Background - Detailed information about using this library
  • Examples - Examples using this library
  • Reference - API reference for this library
• Add a links section to include quick-access links, e.g. issue tracker, source code, PyPi.
• Include note about Python 3.7+ being required for v1.0 and above of this library. Tell people they can keep using an older version of pynrrd if they need to support older Python versions
• Update Makefile for building Sphinx docs to match template