docs(merlin): add In-Network Compute (INC) documentation

[deanchester]

Summary

Adds user-facing documentation for the Merlin In-Network Compute (INC) feature to the SST docs site. INC lets Merlin routers intercept packets in-flight and offload collective operations (e.g. reductions) to per-port accelerator subcomponents, reducing collective latency and bandwidth in HPC fabrics.

This PR is documentation-only — no source or site-infrastructure changes beyond the sidebar wiring.

What's included

• docs/elements/merlin/InNetworkCompute.md — conceptual overview:

  • What INC is and why it accelerates collectives (allreduce/broadcast)
  • How it works: packet interception in PortControl via startINC(), the Accelerator subcomponent API, the router's INC hooks, the incEvent type, and topology awareness (getRtrLevel() / isUpPort())
  • The collective_accel ring accelerator (lport/rport, RING_TYPE modes, timing model)
  • The merlin.inc_nic test endpoint and its parameters
  • Backward-compatibility notes (accelerators are optional; existing sims unaffected)

• docs/elements/merlin/INCQuickStart.md — hands-on guide:

  • Step-by-step walkthrough of fattree_inc_test.py (topology "4,2:4" → 16 hosts, router config, INCJob setup, system.build(), post-build wireAccelerators() ring wiring, running it → 430 ns)
  • INCJob parameter reference, test-suite registration, and an "adapting the example" section

• website/sidebars.js — converts the bare merlin entry into a category object (matching the ember / memHierarchy pattern), keeping intro as the landing page and listing the two new INC pages.

Testing

• Built/served locally with npm run start; both new pages render and appear under the expandable merlin category in the sidebar.
• No new warnings introduced (only pre-existing blog-author / deprecated-config warnings from the site).
• sidebars.js validated via node -e "require('./website/sidebars.js')".

Notes

• Documents the INC feature from the corresponding sst-elements change; no code in this repo is affected.
• Existing merlin/intro.md is unchanged and now serves as the category landing page.