Update AgentChat State Docs (Saving to Disc)

[victordibia]

Why are these changes needed?

Provides explicit example on how to save state to a dict on disc and load it back.

import json


## save state to disk 
with open("coding/team_state.json", "w") as f:
    json.dump(team_state, f)

## load state from disk
with open("coding/team_state.json", "r") as f:
    team_state = json.load(f)

## create team
new_agent_team = RoundRobinGroupChat([assistant_agent], termination_condition=MaxMessageTermination(max_messages=2)) 

## load state
await new_agent_team.load_state(team_state)
stream = new_agent_team.run_stream(task="What was the last line of the poem you wrote?") 
await Console(stream)

Related issue number

Closes #4327

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.