Provide more details on the index sizing calculations

In the Index sizing page https://docs.couchbase.com/server/current/install/sizing-general.html#sizing-index-service-nodes, we could add more details about the overhead memory requires. For example, secondary indexes could require 120 bytes overhead per item between the mainstore and backstore tree structure. This is especially important for MOI, as the indexes will be in memory.

• Possibly separate discussion for MOI vs. Standard/Plasma.
• Add snapshot sizing information needed for MOI disk snapshots.
• Overhead sizing for primary and secondary indexes. In addition to the item data itself, how much overhead is needed per item to maintain the index data structure?
• Considering mainstore and backstore when mentioning sizing.
• Possibly consider adding more information about other (often hard to predict additional sizing needed), such as queues formed due to slow indexing or when an index is first built. Additionally the additional space usage that might come from a delta chain.
• Possibly an honorable mention of the memory usage needed for the Index Service to maintain its basic self ... just to operate ... memory needed for network communications ... etc.

Docs Contrib

Please edit the page linked above to provide the required information. To be backported to 5.0.