docs(crews): document missing params and add Checkpointing section (OSS-32) (#5409)

- Add 8 missing parameters to the Crew Attributes table:
  chat_llm, before_kickoff_callbacks, after_kickoff_callbacks,
  tracing, skills, security_config, checkpoint
- Add new "## Checkpointing" section before "## Memory Utilization" with:
  - Quick-start checkpoint=True example
  - Full CheckpointConfig usage example
  - Crew.from_checkpoint() resume pattern
  - CheckpointConfig attributes table (location, on_events, provider, max_checkpoints)
  - Note on auto-restored checkpoint fields

Closes OSS-32