gh-131323: Add comprehensive key-value type documentation to dbm module

[furkanonder]

• Issue: Missleading example of value validation for dbm since 3.13 #131323

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📚 Documentation preview 📚: https://cpython-previews--137972.org.readthedocs.build/

[eendebakpt]

The sqlite backend accepts integers and floats as keys, but that is not mentioned here.

The structure for the Keys and Values section is slightly different, it would be nice to have them the same. Are there any differences in how keys and values are handled? If not, we might even combine the keys and values sections.

[furkanonder]

Thanks. I've updated the doc to reflect that SQLite accepts int/float (and the other listed types) for both keys and values, an kept a single combined keys/values section since they're handled the same. I also increased test coverage for those key/value types.