Adds examples to AboutArrays

[indented-automation]

PR Summary

Updates the AboutArrays topic with new examples.

Fixes #194

Context

Adds a couple of new examples to AboutArrays, showing Count and briefly discussing + and +=.

Changes

• Adds "arrays are fixed size, elements cannot be added or removed" (not interactive)
• Adds "new arrays are created using the addition operator" (needs tweaking I think)
• Adds "the addition operator can be used to join two arrays together"
• Adds "has properties which describe the size of the array"
• Adds "creates arrays from a range"
• Adds "an array of letters may be created from a range"
• Adds "Object[] and the Array type" (not interactive)

Checklist

• Pull Request has a meaningful title.
• Summarised changes.
• Pull Request is ready to merge & is not WIP.
• Added tests / only testable interactively.
  • Make sure you add a new test if old tests do not effectively test the code changed.
• Added documentation / opened issue to track adding documentation at a later date.

[vexx32]

Looks pretty good! Some minor stuff to tweak, we can sneak in a few extra assertions as well. 🙂

[vexx32]

One last thing I missed and then I think this looks great! 😊