Auto-build docs from markdown files

This patch sets up an auto-build of documentation based on the current set of markdown files in the repo. The basic workflow (/doc/build.sh) is this:

• convert README.md to RST for sphinx
• add a (hidden) toctree to link to each of the other markdown files
• run sphinx-build to generate the docs

To facilitate this, and make the URLs a little nicer the markdown files under /doc/ were renamed (lowercase) and the links in README.md updated to point directly at the published docs, which will be

https://docs.ligo.org/lscsoft/bayeswave/

An example of the docs can be seen here:

https://duncanmmacleod.docs.ligo.org/-/bayeswave/-/jobs/315352/artifacts/doc/_build/html/index.html

(will expire at April 18 2019 at 1427 UK time, please ask me to rerun things and post a new link after that time)