Add Gateway Docs

[significance]

There's lot of info here so I will probably break it down during the docs review, which I will commence as soon as this is merged. I have included, broadly speaking:

• current state of thinking on how and why to run a gateway
• detailed instructions on how to run your own gateway
• an example with instructions using docker compose that runs all three elements and uses the Cloudflare API to automatically provision wildcard TLS certs for free

Would appreciate feedback, I will go through the text once more and also run through the instructions one last time to make sure they work before I merge it.

[agazso]

Nice work, I like it!

I haven't done a full review yet, because it is quite big, but from an overview I would like to point out two things:

• I would avoid mentioning what is planned in the future or what is investigated. I would prefer if the documentation would be about documenting existing things.
• I would avoid mentioning web2 and web3, because they are more marketing terms than technical, therefore their meaning can be highly subjective based on the previous experience of the reader.

[significance]

thanks for your input pushed a much revised version which i think should address most of your feedback

[significance]

@agazso @vojtechsimetka @zelig have completed an in-depth review of the whole thing. much better now.

would welcome your feedbacks