RDoc now has a `--server` mode that starts a local HTTP server
with live reload. Add an `html-server` make target so developers
can preview Ruby documentation while editing source files.
Usage: `make html-server` from the build directory, then visit
http://localhost:4000. Editing any source file's documentation
comment will automatically refresh the browser.
Use `make html-server RDOC_SERVER_PORT=8080` to change the port.