Adjust sphinx settings

Change deprecated config ":show-nested:" setting in favor of ":nested:"
as per used sphinx-click 2.5.0 version.

Remove empty page "celery.bin.amqp.html" ("celery.bin.amqp" only now
has click documentation shown in "reference/cli.html").

Relates: celery#6902 celery#6905

Author: alesolda

docs/reference/celery.bin.amqp.rst

@@ -4,4 +4,4 @@
 
 .. click:: celery.bin.celery:celery
    :prog: celery
-   :show-nested:
+   :nested: full