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

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
```