Document the NetworkIO storage I/O feature

[deanchester]

Summary

Adds user-facing and motif-developer documentation for the NetworkIO
storage I/O feature landing in sst-elements via
sstsimulator/sst-elements#2699.

This PR is documentation only — it introduces no functional change. It
should be merged alongside (or just after) elements PR #2699 so the docs
match the shipped feature.

What NetworkIO is

NetworkIO lets an Ember motif issue file-like storage I/O (read/write,
file handles, non-blocking I/O) to dedicated I/O nodes over the same Merlin
fabric the MPI traffic uses — so simulations can study storage I/O
contending with compute communication. The feature spans four element
libraries (hermes API, firefly implementation, ember motif + tests, merlin
Python wiring); these pages document how to use it end to end.

What's in this PR

New pages

Page	Audience	Contents
docs/elements/ember/NetworkIO.md	User	End-to-end storage guide: wiring a simulation, the TestNetworkIO motif and its parameters/op-modes, I/O pools, striping, async I/O, collecting statistics, and writing your own NetworkIO motif
docs/elements/hermes/NetworkIO.md	Motif developer	hermes::NetworkIO API reference (types + Interface methods), an MPI-IO call-equivalence table, and the intentionally out-of-scope list
docs/elements/firefly/NetworkIO.md	Advanced	Firefly components: hadesNetworkIO, the Block/RR/Hash striping mappers, storage pools (hadesStorageController), and the SimpleSSD timing model

Updated pages

• docs/elements/firefly/intro.md — link to the new NetworkIO page
• docs/elements/hermes/intro.md — link to the new NetworkIO page
• docs/elements/merlin/intro.md — document System.allocateIoNodes()
• website/sidebars.js — wire the three new pages into the sidebar nav
  (promotes firefly and hermes to category blocks so their NetworkIO page
  nests under the existing intro)

Notes for reviewers

• All content is grounded in the source shipped by elements PR #2699 — the
  API surface, motif parameters, mapper defaults, and statistics were read
  directly from the implementation rather than paraphrased from design
  notes.
• The hermes page deliberately marks features that are not yet
  implemented (collective I/O, file views/derived datatypes, individual and
  shared file pointers, typed buffers, sync/atomicity) so readers don't
  expect full MPI-IO parity.
• No node_modules/lockfile changes are included (git-ignored in this repo).

Related

• Tracks: sstsimulator/sst-elements#2699