Add ToC to couchbaseClusterConfig.md

The current cluster config document is very long and it is hard to catch the H2s while scrolling. We should try and add some intra-page navigation, such as a ToC.

Dev Portal version: https://developer.couchbase.com/documentation/server/5.5/operator/couchbaseClusterConfig.html

Beta site version: http://docs.couchbase.com/prerelease/couchbase-operator/beta/couchbaseClusterConfig.html

The Dev Portal version should be auto-generating a right-hand nav for H2s. Amarantha is investigating why this is not happening.

If we can get the Dev Portal navigation working easily, then we will go with that and leave the Beta site without navigation. If we can't get it working, we can investigate a markdown solution that should work for both sites.