[DOP-13251] Add Greenplum type mapping to documentation

[dolfinus]

Change Summary

• Added separated page with Greenplum <-> Spark types mapping, and recommendations related to type casts.
• Describe set of DBReader features supported by Greenplum connector.
• Added more examples to reading & writing data to Greenplum, 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 94.12%. Comparing base (3fe2aa8) to head (0adb357).
Report is 3 commits behind head on develop.

❗ Current head 0adb357 differs from pull request most recent head 454c8de. Consider uploading reports for the commit 454c8de to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #228   +/-   ##
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  Coverage    94.12%   94.12%           
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  Files          204      204           
  Lines         7836     7836           
  Branches      1399     1399           
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  Hits          7376     7376           
  Misses         335      335           
  Partials       125      125           

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