JS Documentation (#3535)

* Initial JS documentation
* Spelling fix

Co-authored-by: あく <alleteam@gmail.com>

documentation/js/js_builtin.md

@@ -0,0 +1,56 @@
+# Built-in methods {#js_builtin}
+
+## require
+Load a module plugin.
+
+### Parameters
+- Module name
+
+### Examples:
+```js
+let serial = require("serial"); // Load "serial" module
+```
+
+## delay
+### Parameters
+- Delay value in ms
+
+### Examples:
+```js
+delay(500); // Delay for 500ms
+```
+## print
+Print a message on a screen console.
+
+### Parameters
+The following argument types are supported:
+- String
+- Number
+- Bool
+- undefined
+
+### Examples:
+```js
+print("string1", "string2", 123);
+```
+
+## console.log
+## console.warn
+## console.error
+## console.debug
+Same as `print`, but output to serial console only, with corresponding log level.
+
+## to_string
+Convert a number to string.
+
+### Examples:
+```js
+to_string(123)
+```
+## to_hex_string
+Convert a number to string(hex format).
+
+### Examples:
+```js
+to_hex_string(0xFF)
+```