Move "Add documentation on OVN BGP agent setup with EVPN" to documentation

[scoopex]

SovereignCloudStack/hardware-landscape#51

This documentation describes the development status for "Add documentation on OVN BGP agent setup with EVPN
". As you can see, there are currently some Openstack change requests waiting to be integrated into Kolla-Ansible.

So that this can be tracked, it makes sense to create a new menu item ‘Technical Previews’ below https://docs.scs.community/docs/category/iaas-layer where this document is then stored.

(https://github.com/SovereignCloudStack/docs/tree/main/docs/02-iaas)