Add documentation for running XAOT with xpk

[Shuwen-Fang]

Description

Document steps to deploy xpk with XAOT to reduce compilation time at runtime.

Tests

Tested by following documented steps to successfully deploy a training run.

Checklist

Before submitting this PR, please make sure (put X in square brackets):

• I have performed a self-review of my code. For an optional AI review, add the gemini-review label.
• I have necessary comments in my code, particularly in hard-to-understand areas.
• I have run end-to-end tests tests and provided workload links above if applicable.
• I have made or will make corresponding changes to the doc if needed, including adding new documentation pages to the relevant Table of Contents (toctree directive) as explained in our documentation.

[gobbleturk]

Thanks Shuwen!