[docs] Add documentation for Map data type

[qzyu999]

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• I searched in the issues and found nothing similar.

Motivation

Add documentation for the newly introduced Map data type. This will likely involve updating api-reference.md and data-types.md to reflect the new data type and its usage, mirroring the documentation patterns established for the Array type and other existing data types.

Solution

Mirror the changes for https://github.com/apache/fluss-rust/pull/433/changes#diff-0015a7f498603263814916639bd0d46b643915bfee9a27814ed19e6b9be7ec83 regarding how the array dtype added to the docs.

Anything else?

No

Willingness to contribute

• I'm willing to submit a PR!