Migrate documentation to ReadTheDocs, drop custom template

This PR splits the documentation into a manually maintained section with intro, quickstart, FAQ and high-level reference, and an auto-generated low-level interface section.  The manually maintained section is under ``docs`` and is compatible with ReadTheDocs via CNAME alias to docs.h5py.org.  It will be updated automatically whenever commits are merged into master.  The auto-generated section is hosted in an S3 bucket at api.h5py.org, and will be updated roughly once per X.Y release.

This should make it easier to maintain up-to-date documentation, encourage user editing via the "Edit on Github" button available at RTD, and reduce the burden on the core team for building & uploading docs.

Also included are a number of fixes and improvements to the documentation content.