Docs: clarify MicroCloud networking requirements and non-multicast initialization options

While MicroCloud supports non-multicast initialization, the documentation on this process is not clear, it is only mentioned in [one paragraph](https://canonical.com/microcloud/docs/latest/explanation/initialization/#automatic-server-detection) in the reference section and doesn't include examples.

We should:

- clarify the [networking requirements](https://canonical.com/microcloud/docs/latest/reference/requirements/#required-network-interfaces) so that it mentions both multicast and non-multicast options. In addition, differentiate that we support the non-multicast for the intra-cluster network used for setting up MC, but the additional network interface to connect to the uplink network which is used for OVN requires a shared L2.
- adjust the [initialization how-to](https://canonical.com/microcloud/docs/latest/how-to/initialize/) so that it explains non-multicast setup too

This could potentially also have its own topic in the explanation section. cc @elijahgreenstein

Start with the linked networking requirements and initialization how-to pages, then review the existing automatic server detection paragraph. Update the documentation with multicast and non-multicast initialization examples, distinguish the intra-cluster and OVN uplink network requirements, and ensure both linked pages explain the supported options clearly.

Written by the indexing model from the issue text.