docs: import Linux kernel-doc script and extension

docs: import Linux kernel-doc script and extension

Import Linux's kernel-doc script as of commit 15e2544ed38a1e, as well
as the Sphinx extension to call kernel-doc according to the arguments
and parameters given to a reStructuredText directive.

The kernel-doc extension accepts a filename, which is relative to
the QEMU source tree root.  The extension also notifies Sphinx about the
document dependency on the file, causing the document to be rebuilt when
the file has been changed.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>