Docs: Fix build preview banner

[hugovk]

There should be a banner at the top of PR build previews for docs. Here's it working on this branch as a PR on my fork:

But for some reason it's missing for this upstream:

TODO

The code to control this:

cpython/Doc/conf.py

Lines 117 to 124 in f0ad456

	# Deployment preview information
	# (See .readthedocs.yml and https://docs.readthedocs.io/en/stable/reference/environment-variables.html)
	repository_url = os.getenv("READTHEDOCS_GIT_CLONE_URL")
	html_context = {
	"is_deployment_preview": os.getenv("READTHEDOCS_VERSION_TYPE") == "external",
	"repository_url": repository_url.removesuffix(".git") if repository_url else None,
	"pr_id": os.getenv("READTHEDOCS_VERSION")
	}

Let's start with a draft PR to debug the env vars.

[hugovk]

It's working here too:

https://cpython-previews--104082.org.readthedocs.build/en/104082/

---

Looking at another PR build that is missing the banner:

• https://cpython-previews--104029.org.readthedocs.build/en/104029/
• https://readthedocs.org/projects/cpython-previews/builds/20353556/
• GH-103963: Make dis display names of args for intrinsics opcodes #104029

The RtD build page says:

Configure your documentation builds! Adding a .readthedocs.yaml file to your project is the recommended way to configure your documentation builds. You can declare dependencies, set up submodules, and many other great features.

And looking at the PR, it's from before the #103843 was merged that updated conf.py, so I think we're good here.

One thing to do though, is backport #103843 to at least 3.11 so we get banners on backport builds too.

[hugovk]

Backport PR: #104083