[DOP-14058] Improve Kafka documentation

[dolfinus]

Change Summary

• Move parse_column and serialize_column examples to docstrings.
• Update reading & writing Kafka documentation to match other connectors - include list of supported DBReader features, code examples, simplify Dataframe schema note.
• Add Prerequisites section to Kafka, describing connection addresses, protocols, auth mechanisms and so on.
• Add Troubleshooting section. Currently it's not much we can mention here, but this can be improved in future versions.

Related issue number

Checklist

• Commit message and PR title is comprehensive
• Keep the change as small as possible
• Unit and integration tests for the changes exist
• Tests pass on CI and coverage does not decrease
• Documentation reflects the changes where applicable
• docs/changelog/next_release/<pull request or issue id>.<change type>.rst file added describing change
  (see CONTRIBUTING.rst for details.)
• My PR is ready to review.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.71%. Comparing base (aa2b753) to head (dccd8c5).

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #276      +/-   ##
===========================================
+ Coverage    94.69%   94.71%   +0.02%     
===========================================
  Files          204      204              
  Lines         8068     8068              
  Branches      1410     1410              
===========================================
+ Hits          7640     7642       +2     
+ Misses         305      304       -1     
+ Partials       123      122       -1     

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