Add Section about 'velocitas init' with package parameter

[wba2hi]

Describe your changes

Add section to velocitas CLI explaining how to install single packages.

Issue ticket number and link

https://github.com/eclipse-velocitas/cli/issues/62

Checklist before requesting a review

• [x] I have performed a self-review of my code
• [x] If it is a core feature, I have added thorough tests.
• [ ] Do we need to implement analytics? No, I guess
• [ ] Will this be part of a product update? If yes, please write one phrase about this update. ???

[wba2hi]

Preview can be seen here: https://github.com/SoftwareDefinedVehicle/velocitas-docs/blob/features/add-velocitas-init-with-package-section/content/en/docs/concepts/lifecycle_management/velocitas_cli.md#velocitas-init