documentation in source tree and on website misaligned

The "compatibility table" in the source tree is complete: `docs/installation` correctly mentions that SUNDIALS version 7.x goes with odes version 3.1.x, but the docs on readthedocs are not up to date at https://scikits-odes.readthedocs.io/en/latest/installation.html and the table stops at sundials 6.x