Update "Contributing" page

[colinmollenhour]

Please assign to yourself if you are claiming this task.

https://www.openmage.org/partners/what-you-can-do.html

• Proof-read, cleanup, improve...

[adam-maciejewski]

Redesigned What you can do page is available on prod: https://www.openmage.org/partners/what-you-can-do.html