We should update the main MR template

I'm raising an issue here, but this can't be fixed in an MR. It has to be updated in the settings in the UI, as far as I can tell: https://docs.gitlab.com/ee/user/project/description_templates.html#setting-a-default-template-for-merge-requests-and-issues--starter

The main issue is that it isn't 100% clear that we always want the docs included with the feature MR, especially now that we have a single codebase. The docs review doesn't have to be a blocker, but the docs themselves should be mandatory (for user facing changes).

If the docs review can't be finished before merge, then merge and raise an issue to clean up the docs afterwards, but always include the docs.

Currently we have (for the documentation item in the template):

- [ ] [Documentation created/updated](https://docs.gitlab.com/ee/development/documentation/feature-change-workflow.html)
  or [follow-up review issue created](https://gitlab.com/gitlab-org/gitlab/issues/new?issuable_template=Doc%20Review)

I suggest:

- [ ] [Documentation created/updated](https://docs.gitlab.com/ee/development/documentation/feature-change-workflow.html),
  (required for all user-facing changes). If docs not reviewed by a technical writer before merge,
  [create a follow-up issue for TW review](https://gitlab.com/gitlab-org/gitlab/issues/new?issuable_template=Doc%20Review)