[DOP-13253] Add Postgres types & prerequisites documentation

[dolfinus]

Change Summary

• Added separated page with Postgres <-> Spark types mapping, and recommendations related to type casts.
• Added separated page with Postgres prerequisites like post number and grants.
• Split documentation of DBReader and Postgres.sql to different pages
• Added more examples to reading & writing data to Postgres, and using fetch and execute methods.

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 94.13%. Comparing base (6849b46) to head (2adc3a4).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #229   +/-   ##
========================================
  Coverage    94.12%   94.13%           
========================================
  Files          204      204           
  Lines         7836     7837    +1     
  Branches      1399     1399           
========================================
+ Hits          7376     7377    +1     
  Misses         335      335           
  Partials       125      125           

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