docs: add MR default template

Related to Ensure SREs Can Determine if Deployed Registrie... (#1020 - closed)

It adds the following default MR template.

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What does this MR do?

%{first_multiline_commit}

Author checklist

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added unit tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines
• For database changes including schema migrations:
  • Run bin/registry database migrate up config.yml successfully.
  • Run bin/registry database migrate down config.yml successfully.
  • TODO we should have checkboxes that it's been tested that it's safe for cny deploys and for rollbacks, with links to documentation on how to do that.

Reviewer checklist

• The commit message and the MR description is still accurate.
• The change is safe to deploy.

If the MR introduces database schema migrations:

• Identify if the change can be rolled back safely. (note: all other reasons for not being able to rollback will be sufficiently captured by major version changes).

If the changes cannot be rolled back follow these steps:

• If not, apply the label cannot-rollback.
• Add a section to the MR description that includes the following details:
  • The reasoning behind why a release containing the presented MR can not be rolled back due to schema migrations.
  • Detailed steps to revert/disable a feature introduced by the same change where a migration cannot be rolled back. (note: ideally MRs containing schema migrations should not contain feature changes.)
  • Ensure a new release is cut with these changes only.