Add canary page

[a-velasco]

• Have you updated CHANGELOG.md with relevant non-documentation file changes?
• Have you updated the documentation for this change?

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Port of changes from a-velasco/sphinx-stack-testing#1 to the DOCPR-2690 feature branch of the sphinx-stack repository. This makes it easier to keep in sync with the Stack and test behaviour that relies on the final RTD URL.

This PR contains changes made after @medubelko's first review. All suggestions were applied.

This PR adds a page with simple tests for every default extension in the Stack that could potentially break.

[a-velasco]

Build failure comes from usage of github_repo in the conf.py in order to make contributors appear in the footer:

WARNING: conf.py setting 'github_folder' is deprecated. Use 'repo_folder' instead.

This warning also comes up if I keep only repo_folder, and if I keep both repo_folder and github_folder. Didn't have time to troubleshoot this in depth today, so unsure if it's a bug with the current Stack or a misconfiguration on my part.

(Contributor footer only correctly renders when only github_repo is used - despite the build warning)

[a-velasco]

Note: I repurposed the "Sphinx Stack testing" RTD project and created a version called agentic-testing.