Default to automatically finding an open port if 8000 is unavailable

[ianhi]

Current Behavior

Default sphinx-autobuild ... will open port 8000, and if that's already occupied it will fail with an error

The HTML pages are in build/html.
[sphinx-autobuild] Serving on http://127.0.0.1:8000
[sphinx-autobuild] Waiting to detect changes...
ERROR:    [Errno 48] error while attempting to bind on address ('127.0.0.1', 8000): [errno 48] address already in use
make: *** [watch] Error 1

This PR

Still defaults to port 8000. But if that port is occupied then fall back to auto detecting a port. I tihnk this makes for a better user experience and is what most users will want. It also mimics the common behavior in the npm ecosystem. But it's sitll possible to request a specific port and have it fail

[sphinx-autobuild] Port 8000 already in use. Attempting to find a free port...
[sphinx-autobuild] Using port 50268 instead.
[sphinx-autobuild] Starting initial build
[sphinx-autobuild] > python -m sphinx build docs docs/_build/html
Running Sphinx v8.2.3
....
The HTML pages are in docs/_build/html.
[sphinx-autobuild] Serving on http://127.0.0.1:50268
[sphinx-autobuild] Waiting to detect changes...

You can still specify a port and have it fail if occupied

➜  sphinx-autobuild git:(auto-ports) ✗ uv run sphinx_autobuild docs docs/_build/html --port 8000
[sphinx-autobuild] Error: Cannot bind to 127.0.0.1:8000. The port is already in use. Use --port 0 to automatically find a free port.