Add initial CALL Clause documentation

docs/clauses/call.md

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+# CALL
+
+The CALL clause is used to invoke a built-in or a user-defined function.
+
+## CALL ... YIELD
+
+The basic syntax for the CALL ... YIELD clause is as follows:
+
+```postgresql
+CALL functionName(arg1, arg2, ..., argn) YIELD functionName
+```
+
+Where `functionName` is the name of the stored or user-defined function to be called.
+`arg1`, `arg2`, ..., `argn` are the arguments to be passed to the procedure or function.
+
+Some examples of how to use the CALL clause:
+
+## Call a Built-in Function
+
+```postgresql
+SELECT *
+FROM cypher('graph_name', $$
+    CALL toInteger('1') YIELD toInteger AS int
+    RETURN int                        
+$$) as (c agtype);
+```
+
+<table>
+  <tr>
+    <td><strong>c</strong></td>
+  </tr>
+  <tr>
+    <td>1</td>
+  </tr>
+  <tr>
+    <td>(1 row)</td>
+  </tr>
+</table>
+
+This example calls the `toInteger()` built-in function to yield the string as an integer.
+
+## Match then Call a Built-in Function
+
+```postgresql
+SELECT *
+FROM cypher('graph_name', $$
+    MATCH (v)
+    CALL label(v) YIELD labels AS l_v
+    CALL properties(v) YIELD properties AS p_v
+    RETURN l_v, p_v
+$$) as (labels agtype, props agtype);
+```
+
+<table>
+  <tr>
+    <td><strong>labels</strong></td>
+    <td><strong>props</strong></td>
+  </tr>
+  <tr>
+    <td>"Person"</td>
+    <td>{"name": "Wes"}</td>
+  </tr>
+  <tr>
+    <td>"Person"</td>
+    <td>{"name": "Doe"}</td>
+  </tr>
+  <tr>
+    <td>"Person"</td>
+    <td>{"name": "Cruz"}</td>
+  </tr>
+  <tr>
+    <td>(3 rows)</td>
+  </tr>
+</table>
+
+This example calls the built-in functions `label()` & `properties()` to retrieve the labels & properties of the matched vertices.
+
+## Calling a User-Defined Function
+
+```postgresql
+CALL myFunc("AGE", "Graph Database") YIELD myFunc
+```
+
+This example calls a user-defined function named `myFunc` and passes the arguments `"AGE"` and `"GRAPH DATABASE"` to the function.