Expose convert_to_rzz_valid_pub

[yaelbh]

In #2126 we introduced a new function convert_to_rzz_valid_pub. Although #2126 is merged, one review comment is still not addressed.

Copying the comment here, for convenience:

Since this is not a transpiler pass that will be run automatically, do you want to expose it in a more public way by including it in the documentation and possibly providing a higher level import path than qiskit_ibm_runtime.transpiler.passes.basis.fold_rzz_angle?

Questions to the repository maintainers:

1. Which import level do you think fits?
2. Where does the relevant documentation reside?