vpn/ipsec: How to use swanctl.conf as migration help from legacy to connections

[Monviech]

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https://github.com/opnsense/core/pull/7972

This was added to ease migrations, a small document how to use it to migrate a policy based tunnel could be created.

[Monviech]

It seems there already is a document for this:

https://docs.opnsense.org/manual/vpnet.html#migrating-from-tunnels-to-connections

So I will add it there.