Add "Alcotezt" merge request description template

Context

This MR adds a merge description template for the %Unit tezts / Alcotezt milestone. It should simplify MR descriptions and reviews by providing detailed instructions on review. Furthermore, using GitLab quick actions it presets a set of labels, the correct milestone and an initial set of reviewers. It sets the assignee to the author.

Manually testing the MR

To test it, push to master of a tezos fork and then open a merge request:

$ git push --force-with-lease arvid-tezos tezos/arvid@alcotezt-mr-template:master
..
$ git push --force-with-lease arvid-tezos 
...
remote: 
remote: To create a merge request for arvid@alcotezt-mr-template, visit:
remote:   https://gitlab.com/nomadic-labs/arvid-tezos/-/merge_requests/new?merge_request%5Bsource_branch%5D=arvid%40alcotezt-mr-template
remote: 
...

Now visit https://gitlab.com/nomadic-labs/arvid-tezos/-/merge_requests/new?merge_request%5Bsource_branch%5D=arvid%40alcotezt-mr-template and apply the template "alcotezt". You can check the preview or create the MR to see that the quick actions are applied correctly.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR