Add missing documentation.

What does this MR do?

Add documentation to Build events on webhooks

Are there points in the code the reviewer needs to double check?

Just check if it was really missing.

Why was this MR needed?

Because I was looking for some documentation in this matter and could not find anything, so I wrote those.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Could not find one