docs: restore branch preview

[agoose77]

This PR is the first step in moving our docs to AWS.

It creates a deployment whose URL leads to the rendered docs. Each branch lives for seven days after creation.

[codecov]

Codecov Report

Merging #1994 (df59870) into main (91e7442) will not change coverage.
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Additional details and impacted files

[agoose77]

OK, now the deployment includes the deployed URL, which is a nice touch.

[jpivarski]

As I said before, I can't verify the correctness of this line-by-line, but I approve of the intention.

The "View Deployment" button is great. Is there any reason, now, for editing the PR description? Even though that works, it's a "moving part" that could fly off its axle and break something (maybe accidentally delete a carefully written PR message).

[agoose77]

Is there any reason, now, for editing the PR description?

Now that you mention it, no. Let's remove that, and increase the robustness.