Update sphinx and furo while removing "Edit on GitHub" hack in the process

[joshuacwnewton]

Description

When installing sphinx via pip install sphinx, it depends on a number of extensions (including sphinxcontrib-applehelp):

(venv_sct) jn/4340-update-sphinx|joshua@monarch:~/repos/spinalcordtoolbox$ pip show sphinx
Name: Sphinx
Version: 4.5.0
Summary: Python documentation generator
Home-page: https://www.sphinx-doc.org/
Author: Georg Brandl
Author-email: georg@python.org
License: BSD
Location: /home/joshua/repos/spinalcordtoolbox/python/envs/venv_sct/lib/python3.9/site-packages
Requires: alabaster, babel, docutils, imagesize, importlib-metadata, Jinja2, packaging, Pygments, requests, snowballstemmer, sphinxcontrib-applehelp, sphinxcontrib-devhelp, sphinxcontrib-htmlhelp, sphinxcontrib-jsmath, sphinxcontrib-qthelp, sphinxcontrib-serializinghtml

The newest version of sphinxcontrib-applehelp requires sphinx>=5.0, yet we're currently pinning sphinx to a 4.x version, which causes the RTD build to fail.

Additionally, we're pinning a 2021 version of furo (presumably to avoid upstream versions unexpectedly changing the appearance of our docs), which also pins an old version of sphinx. So, we need to update furo as well if we want to update sphinx.

In turn, updating sphinx and furo require some additional changes:

• language = None now triggers a warning, so change to 'en'
• extlinks now requires explicitly using %s in the caption instead of ''
• Our old "Edit on GitHub" button hack is no longer needed, as furo now provides this function natively, so we can remove the relevant files. (This fixes one of @jcohenadad's concerns, where the "Edit" button had a confusing logo -- now it's a pencil.)

Linked issues

Fixes #4340.

[mguaypaq]

This looks very reasonable. Thanks for the fix and update!