[DOP-13681] Improve MongoDB documentation

[dolfinus]

Change Summary

Sync MongoDB documentation structure and content with other databases, like Clickhouse, Oracle, Postgres - see #233, #234, #229.

See https://onetl--236.org.readthedocs.build/en/236/connection/db_connection/mongodb/index.html

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 94.39%. Comparing base (02ba65a) to head (3a574a3).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #236   +/-   ##
========================================
  Coverage    94.39%   94.39%           
========================================
  Files          204      204           
  Lines         7880     7880           
  Branches      1401     1401           
========================================
  Hits          7438     7438           
  Misses         317      317           
  Partials       125      125           

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