Docs: Split up Pull Request Builds into a how-to guide and reference (Diátaxis)

[benjaoming]

Refs #9746

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📚 Documentation previews 📚

• User's documentation (docs): https://docs--9679.org.readthedocs.build/en/9679/

• Developer's documentation (dev): https://dev--9679.org.readthedocs.build/en/9679/

[humitos]

"Requesting changes" here until we decide the next steps in the conversation we are having in #9676

Note for the future: we are renaming a file, so that we will require a redirect in place.

[benjaoming]

Note for the future: we are renaming a file, so that we will require a redirect in place.

No files are renamed here, the old file stays in place and a new file is added.

We have in the overall plan for this project to not remove any files, so we don't have to become distracted with file names, folder names nor redirects.

We have decided the next steps 👍

[benjaoming]

This is ready and reviewed.

Site-note: Picked up a conversation about finding a repeatable pattern for "Only available on Sphinx" messages. But that can be introduced in another changeset.

Merging this one...