docs: update contributions.md to include k3d as recommended cluster, add details on e2e test setup, and update kubectl install link. Fixes #1750

[danielhelfand]

Closes #1750

Also documents some cluster setup steps under running e2e test section and updates kubectl install link.

• Either (a) I've created an enhancement proposal and discussed it with the community, (b) this is a bug fix, or (c) this is a chore.
• The title of the PR is (a) conventional, (b) states what changed, and (c) suffixes the related issues number. E.g. "fix(controller): Updates such and such. Fixes #1234".
• I've signed my commits with DCO
• I have written unit and/or e2e tests for my change. PRs without these are unlikely to be merged.
• My builds are green. Try syncing with master if they are not.
• My organization is added to USERS.md.

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[codecov]

Codecov Report

Merging #1867 (ee49139) into master (d3c305c) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1867   +/-   ##
=======================================
  Coverage   82.36%   82.36%           
=======================================
  Files         119      119           
  Lines       16872    16872           
=======================================
  Hits        13897    13897           
  Misses       2284     2284           
  Partials      691      691           

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[github-actions]

This PR is stale because it has been open 90 days with no activity.