docs: add LXD purge step to installation instructions

[token47]

In the page https://canonical-microcloud.readthedocs-hosted.com/en/latest/microcloud/how-to/install/ there are installation instructions.

Turns out it is very very easy to fall into some traps while doing it:

• you may just paste the install command and it will not work because ubuntu comes with lxd 5.0 already installed, it will just say it is installed and do nothing. But you do need to refresh it to version 5.21 otherwise you get an error later during init. [1]
• you may realize that and do a refresh instead, upgrading LXD to 5.21 and you still get an error, because the upgrade leaves behind some problems. What you actually need is to PURGE the 5.0 version an then install the 5.21 version. Which is not obvious at all and very much counter intuitive. [2]

So to solve both issues, you could just add an LXD purge before the LXD install in the instructions.

[1] - the error you get is Error: Failed to get OVN client: Failed to get OVN southbound connection string: Failed to run: ovs-vsctl get open_vswitch . external_ids:ovn-remote: exit status 1 (ovs-vsctl: unix:/var/run/openvswitch/db.sock: database connection failed (No such file or directory))
[2] - the error you get is Error: System "ob76-node10" failed to join the cluster: Failed to update cluster status of services: Failed to join "LXD" cluster: Failed to configure cluster :Failed to initialize member: Failed to initialize storage pools and networks: Failed to update local member project "default": Invalid project configuration key "features.networks.zones"

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Reported from: https://canonical-microcloud.readthedocs-hosted.com/en/latest/microcloud/how-to/install/