feat(add IBM Netezza support): documentation changes for Netezza

[abhishekjog]

SUMMARY

Actual code changes for Netezza support are delivered. Need to update documentation for Netezza support.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Documentation change:
Netezza logo should be visible in README.md
Netezza section should be added in superset documentation

ADDITIONAL INFORMATION

• Has associated issue: Add support for IBM Netezza #15452
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #15475 (3964a0d) into master (6c16f9b) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #15475   +/-   ##
=======================================
  Coverage   77.20%   77.20%           
=======================================
  Files         975      975           
  Lines       50830    50830           
  Branches     6728     6728           
=======================================
  Hits        39245    39245           
  Misses      11370    11370           
  Partials      215      215           

Flag	Coverage Δ
javascript	71.77% <ø> (ø)

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