Add html-serve target to autotools and meson

When working on documentation, I find it useful to spawn an HTTP server
to serve docs for me to consume in a web browser. I went ahead and used
Python's builtin http.server module, but it could also be a run target
which just spawned `xdg-open builddir/doc/src/sgml/html/index.html`,
though I would doubt Windows has xdg-open(1).

The one controversial part of this patch might be using "port" 0, which
asks the kernel to assign an unused port. I'm not sure if that exists on
Windows, or is even desireable. Should we have a build option for the
port? What would a good default be?

--
Tristan Partin
Neon (https://neon.tech)