refactor(docs): don't use dockerhub in the quickstart or concepts doc

[krancour]

Partially addresses #1966

Long-term, I'd like if the quickstart perhaps transitioned to using forks of the https://github.com/akuity/guestbook-deploy repo, but this PR is an interim step that at least eliminates the quickstart's dependency on Docker Hub, thereby dramatically reducing the possibility that any one run through the quickstart fails due to rate-limiting.

fwiw, I have been able to run discovery (with a limit of 20) on the Nginx repo in ECR 10x without being rate-limited, so no one should really encounter rate limits with the Warehouse discovery limit set to 5 and performing discovery on its regular schedule.

This should not be merged immediately after approval because it requires corresponding changes to the akuity/kargo-demo repo.

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 46.30%. Comparing base (b492732) to head (0cc8853). Report is 5 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2188 +/- ## ======================================= Coverage 46.30% 46.30% ======================================= Files 242 242 Lines 16767 16767 ======================================= Hits 7764 7764 Misses 8632 8632 Partials 371 371 ```

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Successfully created backport PR for release-0.7:

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