Documentation: Update to Sphinx 5

[amotl]

Build the documentation with Sphinx 5.

References:

• https://github.com/crate/crate-docs-theme/pull/349

[amotl]

The RTD build log confirms that Sphinx 5 is being used.

Successfully installed Sphinx-5.0.2 crate-docs-theme-0.25.0 docutils-0.16 importlib-metadata-4.12.0 sphinx-copybutton-0.5.0 sphinx-sitemap-2.2.0 sphinxcontrib-applehelp-1.0.2 sphinxcontrib-devhelp-1.0.2 sphinxcontrib-htmlhelp-2.0.0 sphinxcontrib-jsmath-1.0.1 sphinxcontrib-plantuml-0.24 sphinxcontrib-qthelp-1.0.3 sphinxext.opengraph-0.6.3 typing-extensions-4.3.0 zipp-3.8.1

-- https://readthedocs.org/projects/crate-dbal/builds/17430748/

It also seems to render well, no?

-- https://crate-dbal--119.org.readthedocs.build/en/119/