docs: explain task and turn tracing spans

[nima20002000]

Summary

Document the default task_span() and turn_span() hierarchy that wraps Runner workflows, so users can interpret the newer trace shape discussed in #3182.

This adds guidance for:

• how task, agent, and turn spans nest
• how guardrail span parentage can vary by run path
• how custom trace processors can identify task/turn spans before and after export

Test plan

• git diff --check
• make build-docs (passes; emits existing MkDocs/griffe/autoref warnings in generated reference and translated docs)

Issue number

Closes #3182

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass