docs: restructure contributing docs and improve E2E test infrastructure

[coopernetes]

Move developer resources (setup, building, testing, code quality) into
CONTRIBUTING.md and refocus the website's contributing page on project
governance (roles, voting, CLA). Slim down localgit/README.md to a
summary now that CONTRIBUTING.md covers E2E setup in detail.

Rename E2E test repositories from real project names to clearly fake
names (test-owner/test-repo, e2e-org/sample-repo). Add fail-fast
pre-flight checks in E2E setup to detect missing Docker infrastructure.
Add Docker Hub login to CI workflow to avoid pull rate limits. Remove
outdated Mocha/Chai testing page from docs site and fix plugin import
paths.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.34%. Comparing base (6fb63d0) to head (f0c0e57).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1398   +/-   ##
=======================================
  Coverage   81.34%   81.34%           
=======================================
  Files          65       65           
  Lines        4648     4648           
  Branches      792      792           
=======================================
  Hits         3781     3781           
  Misses        852      852           
  Partials       15       15           

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[coopernetes]

@jescalada @kriswest this PR points out the challenges with running these types of tests from a forked PR. Note that e2e.yml workflow is never fired since it's trigger is on pull_request, not pull_request_target.
The latter trigger, which would run the e2e job from a fork, has many documented security problems so it is unsafe to provide access to credentials such as Docker Hub via those jobs.

I'm unsure how to most effectively test these changes other than checking out to a local branch on the repo and re-running the checks in an unrelated PR and referencing those checks in this one.

[coopernetes]

Should probably be merged after #1391 and #1385

[linux-foundation-easycla]

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• ✅ login: coopernetes / name: Thomas Cooper (f0c0e57)