Makefile's `serve` rule checks out submodules, making testing theme changes difficult

@jarrodmillman @stefanv

While working on https://github.com/numpy/numpy.org/pull/668, I noticed again that the Makefile's serve rule calls git submodule update. Since the https://github.com/scientific-python/scientific-python-hugo-theme repo is a submodule in the numpy.org and scientific-python.org repos, this makes it a bit more difficult to test theme changes against these sites, because calling make serve tries to set the theme submodules back to what the "superproject" (in git-submodule terms) expects, rather than what the theme developer may have temporarily checked out in the submodule repository.

Both the numpy.org and scientific-python.org repos' Makefiles have a serve-dev rule. In each one, the only difference from the serve rule is that it adds the argument --disableFastRender (which is a Hugo option that affects how it serves the pages locally). I wonder if it would be appropriate to also use the serve-dev rule to not force the submodules to be updated; rather, we could just print a warning if the submodules aren't what's expected. This would allow the theme developer to use a different checkout in the submodule more easily.

Please let me know what you think about this idea. If it sounds good, I'll update the Makefiles accordingly. Thanks.

See also: https://github.com/numpy/numpy.org/pull/526

Read the numpy.org and scientific-python.org Makefiles, comparing their serve and serve-dev rules and the submodule update command. Check how Hugo is invoked and determine how serve-dev should avoid forcing the theme checkout while warning about mismatches. Done means theme developers can test a different submodule checkout without it being reset.