Doc: Render version/language selector on Read the Docs

[humitos]

Integrate the new Read the Docs Addons into the Python Docs Sphinx theme to render versions and languages selector nicely, using the JavaScript readthedocs-addons-data-ready custom event triggered by the Read the Docs Addons.

References:

• Discord thread: https://discord.com/channels/935215565872693329/1159601953265942589
• Implementation of Addons JavaScript CustomEvent: API: use <meta> to define supported API version and trigger CustomEvent readthedocs/addons#64
• Conversation about using Read the Docs: Host the Python docs on ReadTheDocs docs-community#5
• Example of a similar integration on Read the Docs Sphinx theme: Addons: integrate with new beta addons flyout readthedocs/sphinx_rtd_theme#1526

I'm opening this PR here as a POC to continue the conversation on the Discord thread linked.

Example

This example was built locally on a development Read the Docs instance. The languages and versions shown in the example are:

• translations configured on the development Read the Docs instance
• active, built and not hidden versions on the development Read the Docs instance

It shows readthedocs-selector because it's the branch where I was making these changes. It shows latest, because I forgot to disable it 😄

Note this PR is not linked to any issue yet since it comes from a Discord conversation

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📚 Documentation preview 📚: https://cpython-previews--116966.org.readthedocs.build/

[humitos]

I the Discord conversation some weeks ago, we agreed on these following steps:

• test the selectors for mobile browsers
• add some hardcoded links pointing to the official docs as example
• merge the upstream PR for CustomEvent
• maybe ask for someone else's feedback
• merge the PR on CPython and test on Read the Docs
• adapt / fix anything we find required

I was able to work more on the Read the Docs side and I'm waiting for some PRs to be reviewed, merged and deployed there. In the meanwhile, I updated this PR with some changes required to match the API JSON structure (we are freezing on v1 now and we don't expect breaking changes soon)

[humitos]

The required upstream PRs were already merged and we are going to release/deploy them today in a few hours. I pushed the latest required changes to this PR and tested it locally. Everything works as expected, so we are ready to move to the next step.

@hugovk I'd appreciate a small review/approve here and decide whether or not to ask someone else for feedback at this stage. We can also move forward and merge this PR first and after that ask for feedback on the deployed version of the docs under Read the Docs at cpython-previews project.

[humitos]

The required upstream PRs were already merged and we are going to release/deploy them today in a few hours

This is deployed and can be previewed at https://cpython-previews--116966.org.readthedocs.build/en/116966/

[hugovk]

Thanks, this is looking good, let's merge and iterate!

[miss-islington-app]

Thanks @humitos for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[bedevere-app]

GH-118038 is a backport of this pull request to the 3.12 branch.