Add documentation for site builders

I think documentation for site builders would be very useful, so I'm moving this to the 8.0.x branch and updating the issue summary a bit.

Added a sub-guide for setting up an Elasticsearch cluster to connect to → and updated the issue summary with links.

I've built what I feel is a good starting point for a guide - Using Elasticsearch Connector 8.0.x →

There's more information to fill in, but I think that can be done by myself and other contributors, or requested in follow-up issues. Note that I didn't want to duplicate documentation elsewhere, so where possible I directed readers to more-comprehensive guides.

I'm going to tentatively mark this as Fixed. I think if you notice any important omissions or serious problems with the guide, then feel free to re-open this ticket, but for everything else, a more-focused follow-up issue would be best.

Thanks for your understanding and patience!

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