docs: fix stale contribution and e2e debug instructions

[immanuwell]

A couple contributor docs are stale and trip people up a bit.

hack/README.md still points at master and uses git co, which fails on a normal git setup. DEVELOPMENT.md also points to test/rekt/scenarios and mentions TestMyRektScenario, but neither matches this repo.

How to repro

1. Run git co -b my-backport-branch upstream/release-X.Y
2. See git: 'co' is not a git command
3. Run git symbolic-ref refs/remotes/origin/HEAD and see refs/remotes/origin/main
4. Run test -d test/rekt/scenarios || echo missing
5. Run rg 'TestPingSourceWithSinkRef|TestMyRektScenario' test/rekt test -S

Fix is pretty small: use main, switch git co to git checkout, and align the e2e-debug.sh example with the real test and path. Tiny cleanup, but saves a weird faceplant

[knative-prow]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 51.05%. Comparing base (69b976f) to head (4e55357).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #9154   +/-   ##
=======================================
  Coverage   51.04%   51.05%           
=======================================
  Files         411      411           
  Lines       22132    22132           
=======================================
+ Hits        11298    11300    +2     
+ Misses       9960     9959    -1     
+ Partials      874      873    -1     

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