Add docs for modules and import statements (#4435)

docs/kcl/modules.md

@@ -0,0 +1,59 @@
+---
+title: "KCL Modules"
+excerpt: "Documentation of modules for the KCL language for the Zoo Modeling App."
+layout: manual
+---
+
+`KCL` allows splitting code up into multiple files.  Each file is somewhat
+isolated from other files as a separate module.
+
+When you define a function, you can use `export` before it to make it available
+to other modules.
+
+```
+// util.kcl
+export fn increment = (x) => {
+  return x + 1
+}
+```
+
+Other files in the project can now import functions that have been exported.
+This makes them available to use in another file.
+
+```
+// main.kcl
+import increment from "util.kcl"
+
+answer = increment(41)
+```
+
+Imported files _must_ be in the same project so that units are uniform across
+modules. This means that it must be in the same directory.
+
+Import statements must be at the top-level of a file. It is not allowed to have
+an `import` statement inside a function or in the body of an if-else.
+
+Multiple functions can be exported in a file.
+
+```
+// util.kcl
+export fn increment = (x) => {
+  return x + 1
+}
+
+export fn decrement = (x) => {
+  return x - 1
+}
+```
+
+When importing, you can import multiple functions at once.
+
+```
+import increment, decrement from "util.kcl"
+```
+
+Imported symbols can be renamed for convenience or to avoid name collisions.
+
+```
+import increment as inc, decrement as dec from "util.kcl"
+```