nats: fix cluster examples for the scratch-based default image

[koksharoff]

Follow-up to #2710 / #2491: the variant descriptions are now fixed, but the cluster examples still pass -c /etc/nats/nats-server.conf, which only exists in the -alpine variant. On the default (scratch) image the second server exits with open /etc/nats/nats-server.conf: no such file or directory.

Changes (only nats/content.md):

• use /nats-server.conf in the cluster examples and log output
• replace the deprecated --link with a user-defined network
• stop re-publishing host ports already bound by nats-main

Tested the updated commands verbatim against the 2.15 image: the second server joins the cluster (routes visible in /routez). markdownfmt reports no changes.

cc @bruth

[tianon]

cc nats image maintainers: @derekcollison @wallyqs @bruth @neilalexander @philpennock

(fwiw, we usually skip docker network create in docs for images we maintain and just use a --network some-network placeholder, but this seems fine to me either way)

[wallyqs]

LGTM