Docs: Update tutorial and repo with .readthedocs.yaml

[benjaoming]

As part of #10351, we are updating the tutorial:

• Add a .readthedocs.yaml to the tutorial template repo (let's keep it aligned with our how-to example template for Sphinx)
• Remove the part of the guide that suggests to create one
• Adapt the section about the configuration file: "Manage your configuration with .readthedocs.yaml" or similar

Also useful:

• Feedback in User Feedback on Tutorial from Write the Docs Writing Day #10299

Related:

• Docs: Refactor "Importing your documentation" #10383
• Create .readthedocs.yaml tutorial-template#46

[benjaoming]

The current tutorial is using a layout with docs/source separated. It's pretty unconventional and means we would have to have a variation in the example .readthedocs.yaml for the tutorial.

I would like the tutorial to align with the more popular convention of not having a separate Sphinx source dir, such as docs/source so we don't need to also spill this into the example .readthedocs.yaml, but we can move forwards with a consistency throughout our docs and website UI.