Add more useful snippets

[sadiepowell]

We now have a page for documenting useful config snippets: https://docs.inspircd.org/3/configuration/useful-snippets/

We should expand this with more useful tips.

Suggestions are appreciated.

[progval]

Some random ideas:

• self-hosted kiwiirc
• also maybe via a gateway, if that's still relevant?
• a complete example of the recommended link configuration with other servers
• ditto, but with atheme and anope (including Sasl), either by copying from the main repo or just referencing it
• spam mitigation

[sadiepowell]

@progval I'd rather not have config for external software but the IRCd-side stuff for that sounds good!