Add IPv6 mentions to the seeder guide — radicle.xyz

Add a short description on how to configure IPv6 support during the initial “Configuring Your Node” section and a small reminder in the system service section.

I’m not sure if the wording or the layout are ok, but I hope this may be a starting point to improve the seeder guide.

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 ```
 > In the above example the server listen to any IPv4 interface. If your host
 > supports only IPv6 or if you want to support both IPv4 and IPv6, replace
 > `0.0.0.0` with `[::]`.
 Your node is configured with a file named `config.json` in your Radicle home
 directory. You can get the full path of the config file with the `rad self
 --config` command. Additionally, you can output the current configuration with

 Make sure it fits your needs by editing the file directly, or creating an
 override using `systemctl edit`.
 > The downloaded `systemd` unit file is configured to run your node process
 > as the `seed` user and group, for security reasons. If you set up a different
 > user or group name, or set a passphrase during `rad auth`, you will have to
 > make edits to this file before proceeding.
 > The downloaded `systemd` unit file is configured to run your node process as
 > the `seed` user and group, for security reasons. If you set up a different
 > user or group name, or set a passphrase during `rad auth`, or if you want to
 > listen on an IPv6 interface, you will have to make edits to this file before
 > proceeding.
 When you're ready, you can **enable** and **run** the service: