Review 'Understand Logs and Metrics' doc

[melissawm]

Description

Follow-up to #2232

This PR includes some suggestions for the recently merged document "Understand Logs and Metrics".

A few notes:

• I suggest using the "none" lexer for the code blocks here, since they don't actually represent code but outputs in the log. This causes the output to not be highlighted (as opposed to having distracting colors which are not useful in this context).
• I have updated the links to the code to match the current structure with src/MaxText. I used permalinks, so hopefully this won't have to be done every single time code moves around the repo. However in this case I thought it was important since it's a completely new strucutre and the old links (while they work) might be confusing for readers.

Tests

Docs built successfully locally.

Checklist

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

• I have performed a self-review of my code.
• 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.