Cluster Observability: Add high level designs to tech notes

The following listed items high level design should be added to the [tech notes](https://github.com/cockroachdb/cockroach/blob/master/docs/tech-notes/cluster-observability.md). The goal of this tech note is to give someone not familiar with the feature a high level overview of how it works and pointers to key places in code to look for the code. It should not contain minor details that may change or are easy to figure out from the code.

- [ ] Insights (Gerardo, but down to pair with someone 😄 <- I will pair w/ you! - Xin )
- [ ] Logging (Xin - I can do this one but I thought obs-infra technically owns the logging infra)
- [ ] Audit Logging (Thomas)
- [ ] Hot ranges
- [ ] Networking (Andrii)
- [ ] Key visualizer (Zach)

Jira issue: CRDB-30128