"Watch" vs alternative verbiage in Makefile.

https://github.com/canonical/sphinx-stack/blob/c549bf4c049f874c409ff9ee77928be83f10303a/docs/Makefile#L30

Consider better, more descriptive wording with respect to the verb "watch" -- "Serve, Preview, and Auto-rebuild on change"; This will help to more clearly describe what a contributor might expect to see when running this command. For me, at least, "watch" was very confusing.