docs: Add page for Online Tool.

Author: microbit-carlos

docs/contributing.md

@@ -1,7 +1,7 @@
 ---
 layout: page
 title: Contributing
-nav_order: 5
+nav_order: 6
 ---
 
 # Contributing

docs/online-tool.md

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+---
+layout: page
+title: Online Tool
+nav_order: 5
+---
+
+# Online Tool
+
+An implementation example can be found in the `docs/examples/webtool.html` file
+in the repository and it is hosted in the
+[micro:bit Tech Site](ttps://tech.microbit.org).
+
+This web tool can generate a Universal Hex from an Intel Hex file for micro:bit
+V1 and an Intel Hex file for micro:bit V2.
+
+[Click here to go to the Universal Hex Creator](https://tech.microbit.org/software/universal-hex-creator/){: .btn .btn-purple }
+
+![Online Tool screenshot](img/web-tool-screenshot.png)