clanModules/mycelium: init mycelium

This adds a clanModule for `mycelium`.
`Mycelium` is an IPv6 overlay network written in Rust.
Each node that joins the overlay network will receive an overlay network IP in the 400::/7 range.

An example configuration might look like this in the inventory:

```nix
mycelium.default = {
  roles.peer.machines = [
    "berlin"
    "munich"
  ];
  config = {
    topLevelDomain = "m";
    openFirewall = true;
    addHostedPublicNodes = true;
  };
};
```

This will add the machines named `berlin` and `munich` to the `mycelium` vpn.
And will also set the toplevel domain of the mycelium vpn to `m`, meaning the
machines are now reachable via `berlin.m` and `munich.m`.

docs/mkdocs.yml

@@ -92,6 +92,7 @@ nav:
           - reference/clanModules/matrix-synapse.md
           - reference/clanModules/moonlight.md
           - reference/clanModules/mumble.md
+          - reference/clanModules/mycelium.md
           - reference/clanModules/nginx.md
           - reference/clanModules/packages.md
           - reference/clanModules/postgresql.md