doc: update all documentation build dependencies

[matzf]

Update all the dependencies in the documentation build to latest versions. Notable changes are

• sphinx from 6.1.3 to 7.3.7
• sphinx-rtd-theme from 1.2.0 to 2.0.0
• sphinx-autobuild from 2021.3.14 to 2024.4.16 This pulls in a fairly different set of indirect dependencies now.

Fix small typo in toml snippet in manuals/control.rst, as sphinx now warns about this.

Fix small bug in doc/Makefile; build was failing after "make clean", because the realpath function fails if path (_build directory) does not exist.

[matzf]

This change is