Add danger script to auto label ~docs-only merge requests

What does this MR do and why?

Consider creating a label for docs only issues (technical-writing#631 - closed)

This MR adds a danger script to auto label merge requests as docs-only if they only contain file changes in /doc/ with common file extensions (.md, .jpg, .yml etc).

MR 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 MR.