Add regression testing

[detro]

One way to do this could be:

1. Generate docs against a set of repos
2. Compare generated files with fixtures
3. Fail PR tests in case of mismatch

These tests have to be run as part of PR GH Actions.

[bflad]

This is now covered with some txtar testing from @SBGoods 🚀 . If there are specific places we want additional testing, let's create new issues.

[github-actions]

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