docs: manifests for SSO using ArgoCD Dex, to be used with Kustomize

[bgdnlp]

A redo of #5535, which adds some more info around how secrets should be set, and Kubernetes manifests as examples for those who don't deploy with Helm.

Checklist:

• My organization is added to USERS.md.

[codecov]

Codecov Report

Merging #5647 (541fb13) into master (54f4c26) will not change coverage.
The diff coverage is n/a.

❗ Current head 541fb13 differs from pull request most recent head d792314. Consider uploading reports for the commit d792314 to get more accurate results

@@           Coverage Diff           @@
##           master    #5647   +/-   ##
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  Coverage   47.08%   47.08%           
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  Files         240      240           
  Lines       15050    15050           
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  Hits         7087     7087           
  Misses       7059     7059           
  Partials      904      904           

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