docs: add how to install and use helm plugins #2558

[pnowy]

#2558 Based on discussions from:

#2558
#2789
#1105
#1153

I've spent a lot of time to put everything together to get integration with GCS working. I think it's worth to have documentation for it.

Checklist:

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this does not need to be in the release notes.
• The title of the PR states what changed and the related issues number (used for the release note).
• I've updated both the CLI and UI to expose my feature, or I plan to submit a second PR with them.
• [] Optional. My organization is added to USERS.md.
• [X ] I've signed the CLA and my build is green (troubleshooting builds).

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #3766 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3766   +/-   ##
=======================================
  Coverage   41.59%   41.59%           
=======================================
  Files         116      116           
  Lines       16739    16739           
=======================================
  Hits         6962     6962           
  Misses       8871     8871           
  Partials      906      906           

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[alexmt]

Thank you @phowy!