chore: migrate from make linkcheck and make html to sphinx-build -M

[kratsg]

Sphinx seems to ship sphinx-build with a make-mode option called -M (https://www.sphinx-doc.org/en/master/man/sphinx-build.html) which allows one to supposedly drop the Makefile entirely and rely instead on something like

sphinx-build -M html ./source ./build --fail-on-warning

[kratsg]

This works in scipp-atlas/pyhs3#7