[ci skip]

The current master_doc is really only used to specify the toctree to
set up the navbar in the pydata-sphinx-theme.
By renaming to index.rst the navbar config is preserved and the
links back to the main page point to the original landing page
rather than the contents page, which was mostly empty.

 * Remove reference to algebra in the intro text.
 * Link to linalg tutorial in Further Reading
 * Remove linalg heading + WIP from quickstart.

Replace WIP section with link to well-developed svd tutorial.