Add a usage guide for the checkpoint benchmark framework

[mridul-sahu]

Description

Adds GUIDE.md under checkpoint/orbax/checkpoint/_src/testing/benchmarks/: a
structured usage guide for the checkpoint benchmark framework.

It documents:

• the two-part model — a benchmark generator class (its test_fn runs the
  measured code) plus a YAML config that selects and parametrizes it;
• how to write a benchmark and capture metrics with the one-line measure()
  block (including nested per-step measurement), and how to reuse the built-in
  generators;
• the full config reference (checkpoint_config, mesh_config, benchmarks,
  baselines) and the metrics reference (collectors + the TensorBoard tag
  namespaces, with units);
• the feature set — parameter sweeps, repeats + cross-host aggregation, baseline
  capture/compare, profiler traces / HLO dumps, TensorBoard output;
• running on Pathways and cloud — automatic backend init, the colocated-Python
  load dispatcher (load_with_colocated_python), and the XPK launcher;
• worked recipes, a cheat sheet, and a source map linking each component on
  GitHub.

Docs only — no code changes.

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (alters existing behavior or a public API)
• Docs / tests / internal tooling (no user-facing behavior change)

Checklist

• I have read the contribution guidelines.
• Tests cover this change and pass locally.
• Public APIs and non-trivial functions are documented.
• If this change is user-facing, I have updated CHANGELOG.md