CI: upgrade versions of Sphinx and Furo used for doc build

This should fix the CI failure that was visible on gh-583 due
to `sphinxcontrib.applehelp` at least requiring Sphinx 5.0.

Upgrading the Furo theme may also improve the html output; we're
pulling in almost two years worth of fixes here.

docs/reference/pyproject-settings.rst

@@ -31,7 +31,7 @@ use them and examples.
  A string specifying the ``meson`` executable or script to use. If it is a
  path to an existing file with a name ending in ``.py``, it will be invoked
  as a Python script using the same Python interpreter that is used to run
- ``meson-python`` itself. It can be overrridden by the :envvar:`MESON`
+ ``meson-python`` itself. It can be overridden by the :envvar:`MESON`
  environment variable.
 
 .. option:: tool.meson-python.args.dist

pyproject.toml

@@ -53,8 +53,8 @@ test = [
  'typing-extensions >= 3.7.4; python_version < "3.11"',
 ]
 docs = [
- 'furo >= 2021.08.31',
- 'sphinx ~= 4.0',
+ 'furo >= 2023.5.20',
+ 'sphinx ~= 6.2',
  'sphinx-copybutton >= 0.5.0',
  'sphinx-design >= 0.1.0',
  'sphinxext-opengraph >= 0.7.0',