add section on timers

[Dedekind561]

What does this change?

Address #1115

Common Content?

• [x] Block/s

Common Theme?

Yes - it adds to the js2 section.

Issue number: #1115

Org Content?

Updates Data Groups -> Section 3 -> prep

Module | Sprint | Page Type | Block Type

Checklist

• [x] I have read the contributing guidelines
• [x] I have checked my spelling and grammar with an automated tool
• [x] I have previewed my changes to check the markdown renders as I intend
• [x] I have run my code to check it works
• [x] My changes follow our Style Guide

Who needs to know about this?

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