gh-131323: Add comprehensive key-value type documentation to dbm module (#137972)

* Add comprehensive key-value type documentation to dbm module

* Clarify dbm type support and extend type coverage tests

* Re-clarify traditional dbm backend types and drop dumb bytearray test

* Clarify dbm docs for sqlite3 array storage bytes example

* Clarify dbm.dumb bytearray handling and cover with test

* docs: apply devguide style guide to dbm key and value types

* Correct the claim that dbm keys and values are handled identically

Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>

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Co-authored-by: Pieter Eendebak <pieter.eendebak@gmail.com>