docs: Simplify docs workflow build

[wence-]

Description

The workflow build only runs on main, so cull a load of unnecessary steps.

Additionally, adapt the upload action to:

1. Generate the requisite .nojekyll file
2. Force push to the gh-pages branch

The latter will ensure (eventually, once git gc runs on the server) that the CCCL repo doesn't massively bloat with each docs deployment.

Similar to #1194

Checklist

• New or existing tests cover these changes.
• The documentation is up to date with these changes.

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[miscco]

We once dreamed of a time where each PR would generate a documentation @jrhemstad what do you say?

[wence-]

We once dreamed of a time where each PR would generate a documentation @jrhemstad what do you say?

In rapids we do this by pushing the built site into an s3 bucket that we expose via html. e.g. see https://github.com/rapidsai/cudf/actions/runs/20975326181/job/60292097495?pr=21032#step:13:7664

That avoids polluting the gh-pages branch with commits for every PR.

The official github actions deploy-pages action has a preview option (https://github.com/actions/deploy-pages/blob/main/action.yml) but it is github internal only and has been in alpha preview for at least 4 years. (You can see it working e.g. github/opensource.guide#3583)

[wence-]

#5787 is a better version of this.