Create a monthly-task template for the TW team

Converts the information in https://about.gitlab.com/handbook/engineering/ux/technical-writing/#regularly-scheduled-tasks to an issue template in the technical-writing repository. It's likely not perfect, but we might as well start somewhere - it will hopefully make the monthly chores a little less tedious.

Related to gitlab#281596 (closed) where Mike was doing this month's tasks and found a problem (in my docset) 😊