Docs content (#6792)

* Add documentation to modules, and some constants and types

Signed-off-by: Nick Cameron <nrc@ncameron.org>

* Improve the language reference

Signed-off-by: Nick Cameron <nrc@ncameron.org>

---------

Signed-off-by: Nick Cameron <nrc@ncameron.org>

docs/kcl-lang/functions.md

@@ -0,0 +1,46 @@
+---
+title: "Functions"
+excerpt: "Documentation of the KCL language for the Zoo Design Studio."
+layout: manual
+---
+
+We have support for defining your own functions. Functions can take in any
+type of argument. Below is an example of the syntax:
+
+```
+fn myFn(x) {
+  return x
+}
+```
+
+As you can see above `myFn` just returns whatever it is given.
+
+KCL uses keyword arguments:
+
+```
+// If you declare a function like this
+fn add(left, right) {
+  return left + right
+}
+
+// You can call it like this:
+total = add(left = 1, right = 2)
+```
+
+Functions can also declare one *unlabeled* arg. If you do want to declare an unlabeled arg, it must
+be the first arg declared.
+
+```
+// The @ indicates an argument is used without a label.
+// Note that only the first argument can use @.
+fn increment(@x) {
+  return x + 1
+}
+
+fn add(@x, delta) {
+  return x + delta
+}
+
+two = increment(1)
+three = add(1, delta = 2)
+```