docs: update AssistantAgent documentation with a new figure, attention and warning notes #3602

Job	Run time
check-proto-changes-python	1m 8s
docs (./packages/autogen-core)	1m 32s
docs-example-check (./packages/autogen-core)	2m 58s
format	1m 27s
lint	1m 1s
markdown-code-lint	2m 19s
mypy (./packages/autogen-core)	2m 33s
mypy (./packages/autogen-magentic-one)	1m 18s
pyright (./packages/autogen-core)	1m 41s
mypy (./packages/agbench)	56s
samples-code-check	1m 5s
pyright (./packages/autogen-magentic-one)	1m 3s
mypy (./packages/autogen-ext)	2m 13s
pyright (./packages/agbench)	2m 22s
mypy (./packages/autogen-agentchat)	2m 9s
pyright (./packages/autogen-ext)	1m 3s
mypy (./packages/magentic-one-cli)	1m 30s
test (./packages/autogen-core)	1m 0s
pyright (./packages/autogen-agentchat)	1m 12s
test (./packages/autogen-magentic-one)	2m 0s
pyright (./packages/magentic-one-cli)	54s
test (./packages/autogen-ext)	3m 50s
test (./packages/autogen-agentchat)	1m 5s
codecov (./packages/autogen-core)	8s
codecov (./packages/autogen-magentic-one)	12s
codecov (./packages/autogen-ext)	8s
codecov (./packages/autogen-agentchat)	8s
	38m 55s

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