Docs: List extra requirements for use with Raspberry Pi

When running microcloud on a Raspberry Pi (without microovn) the following error message will appear after a cluster is successfully created with `microcloud init`.
```
Error: Failed adding link: Failed to run: ip link add name lxdfan0-fan type vxlan id 15728640 dev eth0 local 192.168.0.43 dstport 0 fan-map 240.0.0.0/8:192.168.0.0/24: exit status 2 (Error: Unknown device type.)
```
The error is appears because the newer Ubuntu RPi kernels don't include the `vxlan` module ([launchpad issue here](https://bugs.launchpad.net/ubuntu/+source/linux-raspi/+bug/1947628))

The kernel module needs to be installed an enabled for networking in lxd cluster containers to work properly.

To enable the `vxlan` module:
- `apt install linux-modules-extra-raspi`
- reboot
- `modprobe vxlan`

This should be documented somewhere in the microcloud documentation.