Product compat matrix improvements

Make the following improvements to our product compatibility matrix page   # Clarify that this is product compatibility matrix and they should refer to support pages for supported versions - link to support policy page -> https://www.couchbase.com/support-policy/enterprise-software

1. Remove CBS 5.x versions. Just start from 6.5 versions of CBS
2. Remove SG 1.4/1.5 from matrix
3. Double check the [1],.[2[,[3] foot notes, there are two tables in page and a common set of footnotes