Add sphinx.configuration to .readthedocs.yml

[pquentin]

As mentioned in https://about.readthedocs.com/blog/2024/12/deprecate-config-files-without-sphinx-or-mkdocs-config/. There was a brownout yesterday, which is how I noticed this.

[github-actions]

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[github-actions]

The backport to 7.17 failed:

The process '/usr/bin/git' failed with exit code 1

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-7.17 7.17
# Navigate to the new working tree
cd .worktrees/backport-7.17
# Create a new branch
git switch --create backport-2740-to-7.17
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 1f66af588fb8d2a96188674bd9ce23f813318aaa
# Push it to GitHub
git push --set-upstream origin backport-2740-to-7.17
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-7.17

Then, create a pull request where the base branch is 7.17 and the compare/head branch is backport-2740-to-7.17.

[pquentin]

Merged to 7.17 in #2745.