Add the ability to prove inclusion to a share commitment

[evan-forbes]

We have the ability to create a share commitment for PFBs here, but we don't include helper functions to create merkle proofs of inclusion to that commitment, which can be confusing for external teams to have to think about. We should add a helper function (and ideally a diagram + docs) on how to use that function.

not strictly blocked, but blocked by being used by #2570