docs: add example for field_references (#520)

related PR: substrait-io/substrait-java#150

---------

Co-authored-by: David Sisson <[email protected]>

site/docs/expressions/field_references.md

@@ -15,11 +15,78 @@ In Substrait, all fields are dealt with on a positional basis. Field names are o
 
 References are typically constructed as a sequence. For example: [struct position 0, struct position 1, array offset 2, array slice 1..3].
 
-#### Validation
+Field references are in the same order they are defined in their schema. For example, let's consider the following schema:
 
-References must validate against the schema of the record being referenced. If not, an error is expected.
+```
+column a:
+  struct<
+    b: list<
+      struct<
+        c: map<string, 
+          struct<
+            x: i32>>>>>
+```
 
+If we want to represent the SQL expression:
 
+```
+a.b[2].c['my_map_key'].x
+```
+
+We will need to declare the nested field such that:
+
+``
+Struct field reference a
+Struct field b
+List offset 2
+Struct field c
+Map key my_map_key
+Struct field x
+```
+
+Or more formally in Protobuf Text, we get:
+
+```
+selection {
+  direct_reference {
+    struct_field {
+      field: 0 # .a
+      child {
+        struct_field {
+          field: 0 # .b
+          child {
+            list_element {
+              offset: 2
+              child {
+                struct_field {
+                  field: 0 # .c
+                  child {
+                    map_key {
+                      map_key {
+                        string: "my_map_key" # ['my_map_key']
+                      }
+                      child {
+                        struct_field {
+                          field: 0 # .x
+                        }
+                      }
+                    }
+                  }
+                }
+              }
+            }
+          }
+        }
+      }
+    }
+  }
+  root_reference { }
+}
+```
+
+#### Validation
+
+References must validate against the schema of the record being referenced. If not, an error is expected.
 
 ### Masked Complex Expression