Review custom port mapping procedure; evaluate migrating it to its own page

https://docs.couchbase.com/server/7.0/install/install-ports.html#map-custom-ports

The procedure to create custom port mappings was updated in DOC-4989, but wasn't fully reviewed for completeness (i.e. does this procedure work on all platforms? do custom port mappings persist between upgrades to Couchbase Server and/or the underlying OS?).

In addition, we should evaluate if this procedure should have its own page. The content on this page is reference, with a procedure tacked onto the bottom. There are likely several other procedures that involve these ports, and it seems odd that we would only have one of them tacked onto this page.