doc(menu/help) Document extensions and scroll behaviour on wiki, closes openscd#557 (openscd#1015)

Author: danyill

public/md/Extensions.md

@@ -0,0 +1,17 @@
+OpenSCD uses extensions to provide built-in functionality and to allow additional functionality beyond the OpenSCD defaults.
+
+Utilities, IED manufacturers, system integrators or other parties can create extensions and provide an interface for viewing or modifying SCL (including modification of private namespaces).
+
+There are three types of extensions:
+
+1. _Menu_ extensions - shown on the project menu after enabling them.
+2. _Editor_ extensions - shown in the main window after enabling them.
+3. _Validator_ extensions - ensure file compliance with a schema or design rules.
+
+To enable extensions, open the project menu by clicking on the top three lines in the top left, scrolling to the bottom and selecting the _Extensions_ item. 
+Then select and deselect functionality. 
+If you are confident in the source of a custom extension, you can also add an extension from a URL.
+
+> NOTE: Infrequently used or in-development extensions are disabled by default.
+
+> NOTE: Where there are many editor extensions, scroll through them by using Shift + Wheel Mouse button.

public/md/Home.md

@@ -5,6 +5,7 @@ You can find information related to topics like
 - [Installation](https://github.com/openscd/open-scd/wiki/Install-OpenSCD)
 - [Project workflow](https://github.com/openscd/open-scd/wiki/Project-workflow)
 - [Validators used in OpenSCD](https://github.com/openscd/open-scd/wiki/Validators)
+- [Extensions](https://github.com/openscd/open-scd/wiki/Extensions)
 
 as well as features that allow you to manipulate or add specific information to the project covering
 

public/md/_Sidebar.md

@@ -32,7 +32,7 @@
 
 ## 5.4 [Conducting Equipment element](https://github.com/openscd/open-scd/wiki/Conducting-Equipment)
 
-## 5.5 [Conducting Equipment element](https://github.com/openscd/open-scd/wiki/Power-Transformer)
+## 5.5 [Power Transformer element](https://github.com/openscd/open-scd/wiki/Power-Transformer)
 
 ## 5.6 [Logical Node References (LNode)](https://github.com/openscd/open-scd/wiki/Logical-Node-Reference)
 
@@ -107,3 +107,5 @@
 ### 9.1 [Manipulate function type elements with OpenSCD](https://github.com/openscd/open-scd/wiki/Function)
 
 ### 9.2 [Create SPECIFICATION IED from function elements](https://github.com/openscd/open-scd/wiki/Create-Specification-IED-from-functions)
+
+## 10. [Extensions](https://github.com/openscd/open-scd/wiki/Extensions)