docs: Move all CSS files to a subdirectory

This unclutters the top-level docs directory.

Signed-off-by: Andrea Bolognani <abologna@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

docs/css/main.css

@@ -0,0 +1,4 @@
+@import url(../fonts/stylesheet.css);
+@import url(generic.css);
+@import url(libvirt.css);
+@import url(mobile.css);