Add the post-portal tech email to the internet-header docs

[oliverschuerch]

If a new project needs to use the internet-header component, the post-portal team must create a header config in there solution. To let them know, a project is in the need for such a config, they must be informed by email. The idea is to add this email and a short description to our internet-header docs.

### Tasks
- [x] Ask the post-portal team if it's ok to add the email to our internet-header docs and point out, that this information is public for everyone.
- [ ] Add a short description, which information needs to be sent and add the email address as a mailto link (do this only if the post-portal team agrees).
- [ ] https://github.com/swisspost/design-system/issues/3672