Add Main UI script editor docs (openhab#2231)

Signed-off-by: Florian Hotze <[email protected]>

mainui/settings/script-editor.md

@@ -0,0 +1,18 @@
+---
+layout: documentation
+title: Script Editor
+---
+
+# Script Editor
+
+<!-- START MAINUI SIDEBAR DOC - DO NOT REMOVE -->
+The script editor is used everywhere in the UI where some type of configuration file or source code has to be edited.
+It provides syntax highlighting and auto-completion for code and Item names for multiple languages.
+
+When using the script editor, you might find the following shortcuts useful:
+
+- <kbd>Ctrl</kbd><kbd>Space</kbd>: Trigger auto-completion.
+- <kbd>Ctrl</kbd><kbd>Q</kbd>: Show documentation (for JavaScript only).
+- <kbd>Shift</kbd><kbd>Tab</kbd>: Indent less.
+- <kbd>Ctrl</kbd><kbd>/</kbd> & <kbd>Cmd</kbd><kbd>/</kbd>: Comment/Uncomment the current line.
+<!-- END MAINUI SIDEBAR DOC - DO NOT REMOVE -->