fix: add missing module init and update docs

[mesakhcienet]

Description

Adds a missing init.py file to the utils/ckpt_scripts directory and updates the documentation.

• Added __init__.py:
  A missing __init__.py under src/MaxText/utils/ckpt_scripts/ causes ModuleNotFoundError when trying to import from this package.
  This issue is present in the current MaxText pip wheel. Adding the init.py makes the directory a package, resolving the import error.

• Documentation Updates:
  The documentation has been updated to include missing explanations and command-line examples for the checkpoint conversion scripts.

Tests

To verify the fix, we performed the following tests:

1. Testing with the current pip wheel (reproduces the error):
   Installed MaxText using uv pip install maxtext
   The import fails:

((myenv) ) mesakh_google_com@t1v-n-bb13ec2e-w-0:~/maxtext-project/maxtext-cienet$ python -c \ 
"from MaxText.utils.ckpt_scripts import convert_deepseek_family_ckpt as ds_ckpt"
Traceback (most recent call last):
File "<string>", line 1, in <module>
ModuleNotFoundError: No module named 'MaxText.utils.ckpt_scripts'

2. Testing with a local build from this updated version:

• Navigated to the MaxText source folder.
• Built and installed the library manually using uv pip install . .

The import now succeeds without error:

((myenv) ) mesakh_google_com@t1v-n-bb13ec2e-w-0:~/maxtext-project/maxtext-cienet$ python -c "from MaxText.utils.ckpt_scripts import convert_deepseek_family_ckpt as ds_ckpt"
((myenv) ) mesakh_google_com@t1v-n-bb13ec2e-w-0:~/maxtext-project/maxtext-cienet$

These tests confirm that adding the __init__.py file resolves the ModuleNotFoundError.

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.