Improve the Rust documentation for the available custom encoders/decoders

[vincent-herlemont]

Currently, the modules containing the encoders/decoders such as native_model::postcard_1_0 are not visible in the documentation. @leontoeides suggests using this to solve this issue.

Related comment: vincent-herlemont/native_db#139 (comment)

TODO

• Investigate.
• Implement.