Introduce docs/internals/ as a two-layer complement to the user manual:
- README.md: index plus the user-manual vs developer-guide rationale
- communication.md: the psb_halo/psb_ovrl -> psi_swapdata dispatch path,
the psb_comm_handle_type hierarchy and factory, the five MPI schemes
(isend/irecv, neighbor alltoallv, persistent, RMA pull/push), and the
start/wait/sync swap-status state machine
