Document collective IO

Nobody has claimed this yet.

The extent of current documentation:
> The conduit_relay_mpi_io library provides the conduit::relay::mpi::io namespace which includes variants of these methods which take a MPI Communicator. These variants pass the communicator to the underlying I/O interface to enable collective I/O. Relay currently only supports collective I/O for ADIOS. -- https://llnl-conduit.readthedocs.io/en/latest/relay_io.html#relay-i-o-path-based-interface

Some examples use `conduit::relay::io_blueprint::save`, for which I see no MPI-equipped variant and have no symbols in the compiled libraries. It looks like perhaps `conduit_relay_mpi_io_blueprint.hpp` was an attempt to create that, but that header isn't public and I don't see how to access the feature.

The wording is ambiguous about whether one should be able to call collectively (with matching filename) for a non-ADIOS protocol (it just won't actually use MPI-IO to put bytes on disk) or whether that may cause conflict/corruption. In what circumstances must the caller distinguish file names by rank?

Start with the Relay I/O documentation at relay_io.html#relay-i-o-path-based-interface and inspect the referenced conduit_relay_mpi_io_blueprint.hpp header. Determine and confirm how collective I/O behaves for each supported protocol, including when ranks need distinct filenames. Done means the documentation clearly states the supported calls, limitations, and filename guidance.

Written by the indexing model from the issue text.