Improve documentation of routing via varnish

[smlx]

Describe the bug
It's not super clear how to route varnish -> nginx/php. The examples in the documentation e.g. here all route directly to nginx.

I also couldn't find any clear documentation about how varnish is automatically configured to route correctly back to the nginx pod if you start sending it traffic.

To Reproduce
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Expected behavior
It would be great to get an example config that we can point people to.

Screenshots
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Additional context
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