Fix edit button on Read the Docs with stable branch builds

[pradyunsg]

What's happening?

The current logic for computing the GitHub URLs in Furo, when on RTD is:

furo/src/furo/theme/furo/components/edit-this-page.html

Line 24 in 490527b

{% set url = "https://github.com/" + github_user + "/" + github_repo + "/edit/" + github_version + conf_py_path + pagename + page_source_suffix %}

This differs from sphinx-rtd-theme (https://github.com/readthedocs/sphinx_rtd_theme/blob/9899ee4ee2f547f81e51297dc12317f018e62fdd/sphinx_rtd_theme/breadcrumbs.html#L39) and should be fixed to align with that.

Reproducer

1. Build Sphinx documentation set on RTD, with a stable branch or tag.
2. See the edit page button's URL -- it has the commit hash rather than the source ref.

Expectation

The generated URL should point to the source ref.

Code of Conduct

• I agree to follow the Code of Conduct.