Header: Custom Content in flag section

[AgreSanGit]

Which issue does this PR close?

This PR closes #3305

What is the new behavior?

Kirby-flag slot section is now open to accept custom content by using the new directive "*kirbyHeaderCustomFlagSection". Example for new feature is added to the cookbook under "Header" flag with custom content.

Does this PR introduce a breaking change?

• [ ] Yes
• [x] No

Are there any additional context?

Checklist:

The following tasks should be carried out in sequence in order to follow the process of contributing correctly.

Reminders

• [ ] Make sure you have implemented tests following the guidelines in: "The good: Test".
• [ ] Make sure you have updated the cookbook with examples and showcases (for bug fixes, enhancements & new components).

Review

• [ ] Determine if your changes are a fix, feature or breaking-change, and add the matching label to your PR. If it is tooling, dependency updates or similar, add ignore-for-release.
• [ ] Do a self-review.
• [ ] Request that the changes are code-reviewed
• [ ] Request that the changes are UX reviewed (only necessary if your PR introduces visual changes)

When the pull request has been approved it will be merged to develop by Team Kirby.

[AgreSanGit]

Very important Needs UI/UX review according to some styling and layout before getting merged