Revert "docs: prevent build error by rolling back Sphinx version"

This reverts commit 65547c8701004f1a2a9ed9d16f1a372f4bd856e4. There is a
new release of sphinxcontrib-httpdomain [1] that fixes the issues with
Sphinx 4.1.x and later [2]

[1] https://github.com/sphinx-contrib/httpdomain/releases/tag/1.8.0
[2] https://github.com/sphinx-contrib/httpdomain/pull/54