JS Documentation (#3535)

* Initial JS documentation
* Spelling fix

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

documentation/doxygen/index.dox

@@ -15,6 +15,7 @@ The documentation is divided into several sections, with all of them accessible
 - @ref dev_tools - Hardware and software tools for all kinds of programming
 - @ref expansion - Additional modules to expand Flipper's consciousness
 - @ref misc - Various useful pieces of information
+- @ref js - JS-based scripting engine documentation
 
 Aside from the manually-written documentation files, there's also a few automatically-generated ones at the bottom of the sidebar:
 

documentation/doxygen/js.dox

@@ -0,0 +1,18 @@
+/**
+@page js JavaScript
+
+This page contains some information on the Flipper Zero scripting engine, which is based on a modified mJS library
+
+- [Brief mJS description](https://github.com/cesanta/mjs/blob/master/README.md)
+- @subpage js_data_types
+- @subpage js_builtin
+
+JavaScript Modules
+JS modules use the Flipper app plugin system. Each module is compiled into a .fal library file and is located on a microSD card. Here is a list of implemented modules:
+
+- @subpage js_badusb - BadUSB module
+- @subpage js_serial - Serial module
+- @subpage js_dialog - Dialog module
+- @subpage js_notification - Notifications module
+
+*/