Adds in-depth walkthrough for testing a custom package like bbctl

General MR

Summary

I found the first-time experience for this repo a bit confusing when testing out a nonstandard package, so I've written up longer docs with a concrete example .gitlab-ci.yml for the bbctl repository as an example.

• adds TOC
• adds walkthrough for an "i want to test a change to bbctl's CI job specifically" scenario

Thanks @danilo-patrucco for walking me through getting this job to work for repo-sync earlier today.

Relevant logs/screenshots

See the README.md rendered on this branch in the changes view.

Linked Issue

issue

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