[docs] add postgres documentation

[nytai]

CATEGORY

Choose one

• Bug Fix
• Enhancement (new features, refinement)
• Refactor
• Add tests
• Build / Development Environment
• Documentation

SUMMARY

Adds documentation for connecting to postgres

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue:
• Changes UI
• Requires DB Migration.
• Confirm DB Migration upgrade and downgrade tested.
• Introduces new feature or API
• Removes existing feature or API

REVIEWERS

@villebro @willbarrett @dpgaspar

[codecov-io]

Codecov Report

Merging #9766 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #9766   +/-   ##
=======================================
  Coverage   70.81%   70.81%           
=======================================
  Files         586      586           
  Lines       30445    30445           
  Branches     3121     3121           
=======================================
  Hits        21559    21559           
  Misses       8772     8772           
  Partials      114      114           

Flag	Coverage Δ
#cypress	53.72% <ø> (ø)
#javascript	59.06% <ø> (ø)
#python	70.93% <ø> (ø)

Impacted Files	Coverage Δ
superset-frontend/src/SqlLab/actions/sqlLab.js	66.81% <0.00%> (ø)

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