Updating docs to include Windows options for SuiteSparse.

- A number of users are now successfully using the
  suitesparse-metis-for-windows project to build Ceres with SuiteSparse
  on Windows.
- Also fixing missing requirement for sphinx_rdm_theme if building
  the Ceres docs.

Change-Id: I1a36c13166f96743557de2ac3b7c9b84531d1f24