Add parallel support for internal manifolds + derived manifold quantities

In updating the documentation during #1257, limitations were noted on parallelization. Currently, co-dimensional manifolds either passing through a single `VolumeSubdomain` or between two or more `VolumeSubdomain` objects can only be used in serial. It would be nice to make them fully parallel.

Some of the exports and derived quantity assemblies are also serial-only (more details in [the documentation](https://festim.readthedocs.io/en/latest/userguide/running_in_parallel.html)).

It would be a nice QOL to make these features fully MPI-compatible

Start with the parallel-running limitations documented in https://festim.readthedocs.io/en/latest/userguide/running_in_parallel.html and review the documentation changes from #1257. Identify the serial-only manifold, export, and derived-quantity paths, then define and verify MPI-compatible behavior for each.

Written by the indexing model from the issue text.