Prove that all of our colimit diagrams are actually colimits

[TOTBWF]

Issue Description

More or less what the title suggests! This should be pretty easy/mindless, so it would be a good beginner issue. See this for what it looks like for a limit diagram.

To do list:

• [x] Coequalisers
• [ ] Pushouts
• [ ] Coproducts
• [ ] Initial objects