docs(reference): move backend info into reference

The info under the top-level
Backends tab was really part of reference, so move it there.

Then it is close to the other top-level ibis.connect() API docs.

I put all the backends/*.qmd into a subfolder
so that the dropdown menu is not overwhelming

docs/_quarto.yml

@@ -80,7 +80,6 @@ website:
     left:
       - sidebar:getting-started
       - sidebar:concepts
-      - sidebar:backends
       - sidebar:how-to
       - sidebar:reference
     right:
@@ -105,13 +104,6 @@ website:
       contents:
         - why.qmd
         - auto: concepts/*.qmd
-    - id: backends
-      title: "Backends"
-      style: "docked"
-      collapse-level: 2
-      contents:
-        - support_matrix.qmd
-        - auto: backends/*.qmd
     - id: how-to
       title: "How-to"
       style: "docked"
@@ -138,9 +130,16 @@ website:
       style: "docked"
       collapse-level: 2
       contents:
-        - section: Expression API
+        - section: Backend API
           contents:
+            - support_matrix.qmd
             - reference/connection.qmd
+            - section: Specific Backend APIs
+              contents:
+                - auto: backends/*.qmd
+
+        - section: Expression API
+          contents:
             - reference/expression-tables.qmd
             - reference/selectors.qmd
             - reference/expression-generic.qmd
@@ -185,15 +184,15 @@ quartodoc:
     member_options:
       signature_name: short
   sections:
-    - title: Expression API
-      desc: "APIs for manipulating table, column and scalar expressions"
+    - title: Connection API
+      desc: Manage backend connections.
       contents:
         - kind: page
           package: ibis
           path: connection
           summary:
-            name: Connection API
-            desc: Manage backend connections.
+            name: Top-level Backend API
+            desc: Create and manage backend connections.
           contents:
             - name: connect
               dynamic: true
@@ -202,6 +201,9 @@ quartodoc:
             - name: set_backend
               dynamic: true
 
+    - title: Expression API
+      desc: "APIs for manipulating table, column and scalar expressions"
+      contents:
         - kind: page
           path: expression-tables
           package: ibis.expr.types.relations