Write the subsection "Multi-Node Training"

[qgallouedec]

This section must be written, with a simple code example, and a link to the accelerate documentation

[SSusantAchary]

I would like to take this. My Plan: add a concise Multi-Node Training subsection with a minimal accelerate example (2 machines, multi-GPU), an accelerate config YAML, env vars overview, and links to Accelerate docs.

I will run pre-commit and keep scope to this page only