Document NT protobuf/struct support

[sciencewhiz]

https://docs.wpilib.org/en/latest/docs/software/telemetry/telemetry.html

While NetworkTables does not yet support serialization of complex data types (this is tentatively scheduled for 2024), mutable types from user code can be easily extended to interface directly with WPILib dashboards via the Sendable interface, whose usage is described in the next article.