Provide documentation for DataTable columns that are JSON encoded

[ddelnano]

Pixie's data collector inserts records into the following DataTables. The telemetry in these tables is a mix of protocol traces (http_events, kafka_events), metrics, and profiling information. Most data table columns serve a single purpose, but some contain JSON-encoded strings to accommodate various request/response data use cases.

For example, the tls_events table may contain traces of TLS handshakes, TLS alerts, or TLS heartbeats. Similarly, the kafka_events table includes different request and response bodies depending on the API version (e.g., produce, consume, etc.).

Unfortunately, the structure of these JSON strings is undocumented, requiring users to analyze each protocol's parsing code in detail. Pixie users often seek to extract deeper protocol insights from these JSON columns, making this a frequent topic of discussion in our community Slack.

It would be valuable if our documentation surfaced the structure of these strings, streamlining users' access to protocol-specific telemetry and reducing the need for extensive code analysis.