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 https://github.com/scipp-atlas/pyhs3/pull/7