[DOP-9085] Improve Greenplum documentation

[dolfinus]

Change Summary

Drastically improve Greenplum documentation:

• Added information about network ports, grants, pg_hba.conf and so on.
• Added interaction schemas for reading, writing and executing statements in Greenplum, using PlantUML.
• Added recommendations about reading data from views and JOIN results from Greenplum.
• Removed upper version limit from Sphinx

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

Merging #154 (1f82680) into develop (b8bd6dd) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #154   +/-   ##
========================================
  Coverage    94.24%   94.24%           
========================================
  Files          209      209           
  Lines         7910     7910           
  Branches      1483     1483           
========================================
  Hits          7455     7455           
  Misses         332      332           
  Partials       123      123           

Files	Coverage Δ
...l/connection/db_connection/greenplum/connection.py	70.38% <ø> (ø)
...netl/connection/db_connection/greenplum/options.py	100.00% <ø> (ø)
...nnection/file_df_connection/spark_s3/connection.py	92.56% <ø> (ø)