docs: include full name of signatures

Now the signatures appear as
`ibis.random()` instead of just `random()`.
Per
#7159 (comment)

Before, quartodoc wasn't even respecting
this option, because it was incorrectly
definied at the wrong nesting level.
I had to move the option out of the "member_options" section
for any change to occur.

docs/_quarto.yml

@@ -185,8 +185,7 @@ quartodoc:
   render_interlinks: true
   renderer: _renderer.py
   options:
-    member_options:
-      signature_name: short
+    signature_name: full
   sections:
     - title: Expression API
       desc: "APIs for manipulating table, column and scalar expressions"