Revert "docs: use repository README.rst as doc landing page"

This reverts commit 6e48567811286a6579f0a682a623a093fc7d4161. It
seems http://readthedocs.org/ directly calls sphinx-build without
running the Makefile. Thus we will have to directly copy the README.
This is probably for the best anyway, as we'll want to have a separate
repository landing page and first documentation page.
docs: copy README.rst as new docs/index.rst
docs: update text a bit