Add guidance on other robot parts

[antoinepetty]

Some teams don't know what's needed or where to get things like motors, wheels etc

[PeterJCLaw]

From Slack:

In terms of suppliers, teams often use the following:

• https://www.robotshop.com/
• https://uk.rs-online.com/web/
• https://uk.farnell.com/
• https://www.amazon.co.uk/
• https://www.ebay.co.uk/

For parts we would probably suggest:

• At least 4 x 12v motors
• A few servo motors
• A few microswitches for detecting if you bump into something
• Ultrasound sensor
• Wheels & bearings

For the other physical materials, it really varies between teams.
They're often cardboard, MDF, Aluminium, acrylic.

[PeterJCLaw]

I think this is fixed now that the robots 101 series has made it into the docs #527?