Change template for file unreleased.md

[ckunki]

Currently, the template contains only # Unreleased

Requested

# Unreleased

## Summary 

See also

• Add code name to release title #483
• Link in Project attribute "IntRef"

Thoughts:

• The term "Code name" may not be self-explanatory.
• Maybe it's worth a discussion, if we deliberately should deviate from this convention used in our Java projects
• Alternatives could be
  • Release title
  • Release suffix

[ArBridgeman]

Note that this overlaps slightly with #483