[DOP-13252] Improve Oracle documentation

[dolfinus]

Change Summary

• Added separated page with Oracle <-> Spark types mapping, and recommendations related to type casts.
• Added separated page with Oracle prerequisites like port number and grants.
• Split documentation of DBReader and Oracle.sql to different pages
• Added more examples to reading & writing data to Oracle, and using fetch and execute methods.
• Also updated a bit documentation of other DB connections, like Clickhouse, Greenplum, Postgres, to provide unified document structure.

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 (f306a97) to head (27a5f25).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #233   +/-   ##
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  Coverage    94.39%   94.39%           
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  Files          204      204           
  Lines         7880     7880           
  Branches      1401     1401           
========================================
  Hits          7438     7438           
  Misses         317      317           
  Partials       125      125           

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