[documentation] Preview the documentation or merge requests with ReadTheDocs

ReadTheDocs, besides building and hosting documentation, also offers preview of builds. This means that if you have a ongoing work on the documentation, you should be able to preview the website with your changes. That's useful for users contributing for the first time, and not familiar with the syntax, and also for developers to collaborate on the docs.

I have used it with GitHub, but their documentation states: “Only available for GitHub and GitLab currently. Bitbucket is not yet supported.”: https://docs.readthedocs.io/en/stable/pull-requests.html

After RTD builds the Sphinx documentation, the preview URL follows this pattern: https://{PROJECT_NAME}--{PULL_REQUEST_NUMBER}.org.readthedocs.build/en/${PULL_REQUEST_NUMBER}/index.html

New commits trigger new builds, and the preview is updated within seconds or minutes. Here's an example project that uses it:

• https://github.com/common-workflow-language/cwltool/blob/main/.readthedocs.yml