Migrate the documentation to readthedocs

[philthompson10]

Tasks needed to migrate the documentation to readthedocs:

• decide whether to use the existing style or use the more standard one we are using for MetaSIP
• rename the directory from doc to docs and update MANIFEST.in
• update conf.py to pick up the version number from setuptools_scm
• review the documentation for legacy URLs etc
• add a Release Notes section and remove the NEWS file
• update README.md