Improve text of MR empty state

This is an actionable insight from the UX Department: MRs experience async critique research (insight).

What

The text of a merge request's empty state could be improved on the following points:

1. "Interested parties" is a really stuffy, formal term. It also seems a little out-of-place and vague since the rest of the message is directed to "you." Interested parties sounds like it could range from my team members to random GitLab users.
2. It talks a lot about commits, but maybe I don't know what a commit is or how it relates to creating a file.
3. At the end of the message, the calls-to-action don't have buttons. It says “push new commits or use a different branch” but the only button is “Create file”.
   • To be addressed in #291999 (closed)