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)