Add a way to modify the antsibull-docs repository used when installing antsibull-docs

[felixfontein]

Allows to use branches form other forks of antsibull-docs.

I'm using another branch (https://github.com/ansible-community/github-docs-build/compare/main...felixfontein:github-docs-build:repo) to test this in community.dns (https://github.com/ansible-collections/community.dns/commit/075d3a098438e54fffb3630c75b698fd237df1ed, https://github.com/ansible-collections/community.dns/pull/23).

[felixfontein]

could we add this to the tests for the init action?

I'm not sure how to properly do this, except by passing in the value ansible-community/github-docs-build and not acting on it (since you can only see from the pip install stdout where exactly antsibull-docs was installed from), or by passing an empty string to see whether the || works.

(Well, one way to test this would be to have a canary branch in the antsibull-docs repo which provides something else than antsibull-docs; then one can check whether that something else has been installed instead of antsibull-docs. But I don't really like the idea of having something like that in the antsibull-docs repository.)

[briantist]

could we add this to the tests for the init action?

I'm not sure how to properly do this, except by passing in the value ansible-community/github-docs-build and not acting on it (since you can only see from the pip install stdout where exactly antsibull-docs was installed from), or by passing an empty string to see whether the || works.

(Well, one way to test this would be to have a canary branch in the antsibull-docs repo which provides something else than antsibull-docs; then one can check whether that something else has been installed instead of antsibull-docs. But I don't really like the idea of having something like that in the antsibull-docs repository.)

Ok these are good points, thanks.

We can put the canary branch in this repo, but it's probably overkill.

If we keep the || / empty string functionality, testing that might be useful if it's not too difficult. Will trust your judgement on the cost/benefit of that :)

[felixfontein]

Now that || is gone, I think this is ready.

[felixfontein]

Just re-run my branch with https://github.com/ansible-collections/community.dns/pull/23, it still works.

[felixfontein]

@briantist thanks for reviewing!