Information about documentation in docs

[ots22]

• Read the docs builds here: https://empiarreader.readthedocs.io/en/latest/
• PRs make a preview of how the docs will look on RtD
• Build docs by hand in usual way for sphinx (NB these instructions may be wrong):
  • make sure pip install empiarreader[doc] is installed (or pip install .[doc] if installing locally)
  • cd into docs
  • make html
  • open index.html in the build directory
• TODO - note the above in some developer docs