Update Documentation template to reflect standard

Per @aqualls's suggestion, I'm creating this MR to update the Documentation MR template in this repo to be consistent with the Documentation template in the other GitLab repos with documentation.

I left the default label assignments you had included for your stage/group, but if there are other changes that need to be made based on your group's processes or tooling, please let us know.

Issue link: technical-writing#210 (closed)