Allow to serve site with path prefix

[ffplexim]

For testing/debugging purposes (JavaScript problems, absolute vs. relative paths, ...), it can be useful to serve the page with a path prefix. This can be achieved by replacing "/" with "/prefix/" in

sphinx-autobuild/sphinx_autobuild/__main__.py

Line 105 in c9b7cb9

Mount("/", app=StaticFiles(directory=out_dir, html=True), name="static"),

(+ some more polishing, like making sure that the web browser is opened at the right URL etc.)

Is this a feature that's of interest to others?