fix: simplify install instructions

[msvticket]

The current instructions doesn't work with new versions of cosign. Also the current instructions makes for an unnecessary obstacle for new users. So I think simplifying the instructions makes most sense. I'm adding homebrew instructions while at it.

Possibly https://jenkins-x.io/v3/admin/setup/jx3/ should be complemented with cosign instructions.

[jenkins-x-bot]

[jx-info] Hi, we've detected that the pipelines in this repository are using a syntax that will soon be deprecated.
We'll continue to update you through PRs as we progress. Please check #8589 for further information.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 34.62%. Comparing base (762cced) to head (7a7d52b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #8661   +/-   ##
=======================================
  Coverage   34.62%   34.62%           
=======================================
  Files          11       11           
  Lines        1242     1242           
=======================================
  Hits          430      430           
  Misses        767      767           
  Partials       45       45           

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This pull-request has been approved by: tomhobson

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[jenkins-x-bot]

Failed to merge this PR due to:

failed merging [8661]: [Method Not Allowed]