Add documentation for Danger bot project access token

What does this MR do and why?

Documents a new project access token that was required by: !4744 (merged), as discussed in: !4744 (comment 1888780732).

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.