Upgrade documentation loops back on itself

The documentation pages on how to perform an upgrade loop back on themselves and never refer how to actually perform an upgrade of a node.

Starting at https://docs.couchbase.com/server/6.0/install/upgrade.html, we then go to the strategy we want (https://docs.couchbase.com/server/6.0/install/upgrade-online.html). This page then links to the single node upgrade page (https://docs.couchbase.com/server/6.0/install/upgrade-individual-nodes.html) which then recommends you read the server installation page for the OS of use for info on how to upgrade a specific node (https://docs.couchbase.com/server/6.0/install/rhel-suse-install-intro.html). This page then finally redirects you to the initial upgrade documentation for instructions on how to upgrade.

In the installation page, there is no information on the actual upgrading of an existing installation at all on here despite other pages leading to it. We should either explain how to perform the upgrade with yum on the installation page itself (Performing an installation of an already existing package counts as an upgrade for yum, we just don't mention that at all), or include it within the single node upgrade documentation.