Fix footnote styles to work with latest Sphinx + docutils

[jab]

What's happening?

A new version of Sphinx has now been released, which changes its dependency pins to allow docutils 0.19, which includes a fix for New HTML markup for footnotes is difficult to stylise, along with corresponding Sphinx changes.

Now that both Sphinx and docutils have made stable releases to address this issue, I believe Furo is no longer blocked from writing styles that fix the regression in footnote alignment that was caused by the changes in docutils 0.18. (The latest version of Furo is still causing the broken looking footnotes that are visible in this screenshot when used with latest Sphinx (5.1.1) and docutils (0.19).)

Reproducer

1. Build Sphinx documentation at https://github.com/jab/bidict by running pip install -r requirements/docs.txt && make -C docs html from a fresh clone.
2. Open docs/_build/html/index.html in a browser.
3. Scroll down to the footnotes at the bottom of the page.
4. Observe that (1) footnotes are no longer appearing on the same line as their corresponding footnote numbers, and (2), they're not even being indented consistently: Note how footnote 3 is not indented, while footnotes 1, 2, and 4 are indented:

Ref: #452

Expectation

Alignment of footnotes stays consistent and correct, as it appeared before the changes introduced in docutils 0.18 caused this breakage:

Code of Conduct

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