Add guides on Getting Started with JS (#4150)

- Get started section added to the JS docs
- Small fixes in the JS modules docs

documentation/js/js_math.md

@@ -1,6 +1,7 @@
-# js_math {#js_math}
+# Math module {#js_math}
+
+The module contains mathematical methods and constants. Call the `require` function to load the module before first using its methods:
 
-# Math module
 ```js
 let math = require("math");
 ```