Docs: Improve SFT/RL user experience

[hengtaoguo]

Description

Reduce user friction in SFT/RL and fix broken links.

b/463394566
b/463409639
b/463409807
b/463396352
b/463393644

Tests

N/A

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.

[SurbhiJainUSC]

This will be wrong if user sets MAXTEXT_CKPT_PATH in the above sction as it includes 0/items. Maybe we can format this section like this: https://github.com/AI-Hypercomputer/maxtext/blob/8c3289731a28524b631ec62dfb226a357e6e72db/docs/tutorials/posttraining/sft.md#get-your-model-checkpoint to explicitly call this out?

[hengtaoguo]

If the user sets path including 0/items, then the checkpoint will be saved to 0/items/0/items (example). We still need to use as ${MAXTEXT_CKPT_PATH}/0/items?

[hengtaoguo]

Now I see your point and the issue. The previous doc provides two ckpt conversion examples, but the base_output_directory format are different. We should stick to ${BASE_OUTPUT_DIRECTORY}/${RUN_NAME}, and then the actual checkpoint was saved to MAXTEXT_CKPT_PATH=${BASE_OUTPUT_DIRECTORY}/${RUN_NAME}/0/items. We use MAXTEXT_CKPT_PATH to load for SFT later.

https://screenshot.googleplex.com/AsY59zep5hZxNZL

[SurbhiJainUSC]

If user sets MODEL to a different model name other than `Llama3.1-8B, then the following overview section would be misleading. Please rephrase this overview section accordingly.

[hengtaoguo]

Done!

[SurbhiJainUSC]

Is this relevant to this doc?
We already have a different document that provides steps to run MaxText with XPK: https://maxtext.readthedocs.io/en/latest/run_maxtext/run_maxtext_via_xpk.html.

[hengtaoguo]

Yes then I will remove this part and instead points to the xpk doc.