Update layer-system.md

[juhonykanen]

Add numbering to the global layers -list to match the numbers in the illustration below.

[outsystemsbot]

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[juhonykanen]

Hello @juhonykanen 👋

Thanks for your Pull Request. It looks like this may be your first contribution to an OutSystems Open Source project. Before we look into your contribution, we need you to accept the 📝 OutSystems Contributors License Agreement.

After you read and accept the terms of the agreement, reply to this message saying I read, understood and accept the OutSystems CLA.

I read, understood and accept the OutSystems CLA