insights: add 4.0 pings to our pings documentation

Created by: Joelkw

Important to do this while it's still fresh and so we have a SSOT reference in the future! Here is the docs page: https://docs.sourcegraph.com/dev/background-information/insights/code_insights_pings

We can just add them to the bottom. Perhaps it might be nice to add them as a separate #section since they are all related to this rather separate feature.

/cc @joelkw @felixfbecker @vovakulikov