fix general guides landing page

[ivrolan]

What does this PR do?

Fixes #5138 by fixing the location of ../guides_basic and ../guides_advanced with respect the location of index.rst. The link https://flax.readthedocs.io/en/latest/guides/index.html is still used in some other guides like https://github.com/rcrowe-google/Learning-JAX , so it would be nice to keep the link updated.

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Checklist

• This PR fixes a minor issue (e.g.: typo or small bug) or improves the docs (you can dismiss the other checks if that's the case).
• This change is discussed in a Github issue/discussion (please add a link).
• The documentation and docstrings adhere to the documentation guidelines.
• This change includes necessary high-coverage tests. (No quality testing = no merge!)

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[vfdev-5]

Thanks for the PR @ivrolan !
Nice catch and the fix!