docs: add Sphinx configuration to generate Python API documentation

The libvirt-python bindings lack official API documentation despite all the infrastructure for it already existing: the libvirt C project ships XML API descriptions (libvirt-api.xml) with detailed function documentation, and generator.py already extracts these descriptions into Python docstrings in the generated code. However, there was no tooling to render these docstrings into browsable documentation.

Add a basic Sphinx setup with autodoc to generate HTML API reference from the existing docstrings for all four modules: libvirt (main API), libvirt_qemu, libvirt_lxc, and libvirtaio.

Signed-off-by: Radoslaw Smigielski rsmigiel@redhat.com