[train] Ray Train Metrics Doc Page

[JasonLi1909]

This PR:

• adds a new page to the Ray Train docs called "Monitor your Application" that lists and describes the Prometheus metrics emitted by Ray Train
• Updates the Ray Core system metrics docs to include some missing metrics

Link to example build: https://anyscale-ray--58235.com.readthedocs.build/en/58235/train/user-guides/monitor-your-application.html

Preview Screenshot:

[gemini-code-assist]

Code Review

This pull request intends to add a new documentation page for Ray Train metrics. However, the newly created file doc/source/train/monitoring-your-application is essentially empty, containing only a label. It lacks the documentation content described in the pull request description. Additionally, the new file is not referenced in any toctree, so it would not be rendered in the final documentation. The change appears to be incomplete.

[alanwguo]

thanks a lot!