discrete.py docs updated

[androemeda]

Key changes:

1. Updated the initial description to state "values range from 0 to K-1" instead of "1 to K".
2. Added a note explicitly stating that observed values should use 0-based indexing.
3. Updated the mathematical notation to reflect 0-based indexing (k = 0 to K-1).
4. Modified the example to use 0-based indexing for y (0, 1, 2) instead of (1, 2, 3) - 1.
5. Added a comment in the example to highlight the 0-based indexing of y.

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📚 Documentation preview 📚: https://pymc--7442.org.readthedocs.build/en/7442/

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