Add a guide for MaxText checkpointing solutions.

[copybara-service]

Add a guide for MaxText checkpointing solutions.

[melissawm]

Hi all - I left a number of comments.

One high-level comment is that these files are not yet integrated into the documentation pages, as they are not accounted for in any Table of Contents. Because I identified a number of sections which seem to be common between all types of checkpointing, my suggestion would be to have a "Checkpointing guide" parent page, with a toctree directive listing the child pages (emergency_checkpointing, gcs_checkpointing and multi_tier_checkpointing) and maybe containing some of the common sections identified in these files.

If we decide against creating this new page, these three files need to be added to the toctree here: https://github.com/AI-Hypercomputer/maxtext/blob/main/docs/guides.md

One final comment: it is unusual to see capitalization in file names. It is of course up to the team to decide what is best but I just wanted to mention in case it's an oversight.

Thanks!