[1.13] Release-only improvements to functorch docs (#86693)
This PR:
- corrects the functorch version string (previously it said "nightly",
it should instead say the version, which is 1.13).
- Adds permalinks to colabs. The colab notebooks previously linked to
the ones on the master branch, instead, they should link to the ones on
the release/1.13 branch.
Test Plan:
- build and tested docs locally
