Updates to the ReadtheDocs integration

[wmamills]

This PR contains two commits:

• New process for RTDs projects using the new "git checkout command". This commit and the described RTDs project setting updates fix the sub-module PR build process and well as make it simpler.
• Update to Ubuntu 26.04 and Python 3.14 (to keep things current)

As of April or May 2026, ReadtheDocs has changed something in their infrastructure that broken the way were doing sub-module PRs. Luckily, they have added a way to customize the git checkout command used by each RTDs project. This first commit changes things so we rely on that new setting.

The sub-module checkout procedure is still overly complex as RTDs currently lacks an ENV var that easily describes how to checkout the commit needed for each build. However the project settings needed are documented in comments in the .readthedocs.yaml file.

This has been tested (with Bill's repo forks) with latest open-amp lib and openamp-docs as of 2026-05-20 2PM UTC. The open-amp sub-module was tested for pushes to main and for a PR.

NOTE: The RTDs build checks for this PR will fail until I update the RTDs project for the main openamp-docs. I will do this after I get consenses on the patch.

[wmamills]

[arnopo]

LGTM

[arnopo]

@sipke: could you have a look and give your feedback please?
Please also have alook to OpenAMP/open-amp#686

[wmamills]

To document what we said on the meeting May 20, this can be merged before or after the tag of v2026.04.0
Once the RTDs project is updated with the new instructions it will still work with older versions of openamp-docs. The older version will re-checkout the sub-modules in the post_checkout step but they will use the same versions so it will be no-op.

Doc builds for PRs of maintenance branches for sub-modules (like doing libmetal 2025.10.2) wont work correctly but won't fail either. Maintenance for a libmetal 2026.10.2 (after the elimination of the post_checkout job) would build the correct version of libmetal but su-modules PRs will also use the main branch of openamp-docs. Again this only effects doc builds for sub-module builds. All builds from openamp-docs will build the right thing.

[sipke]

The ubuntu change looks fine.
Removing readthedocs config from submodule is fantastic.
The main change of new "git checkout command" is mentioned in PR and the comment add detail, but where is this git checkout command actually happening? or is that what the last NOTE: of the PR is saying that there is more changes incoming?

[wmamills]

Hi @sipke, the change goes into the project settings for each repo in the ReadTheDocs control panel.
I will attach screen shots of what this looks like for my profile and my fork of open-amp lib

my profile:

My open-amp fork project, the settings is on the far right:

The settings main panel look like this:

[sipke]

Understood and now that I do, looks perfect, much cleaner all and all than before.