docs: update AssistantAgent documentation with a new figure, attention and warning notes #3621

Job	Run time
format	3m 50s
pyright (./packages/autogen-core)	1m 19s
pyright (./packages/autogen-magentic-one)	56s
lint	51s
pyright (./packages/agbench)	55s
pyright (./packages/autogen-ext)	1m 5s
pyright (./packages/autogen-agentchat)	1m 7s
check-proto-changes-python	1m 8s
pyright (./packages/magentic-one-cli)	58s
docs (./packages/autogen-core)	1m 43s
docs-example-check (./packages/autogen-core)	3m 1s
markdown-code-lint	2m 1s
mypy (./packages/autogen-core)	2m 37s
mypy (./packages/autogen-magentic-one)	1m 26s
samples-code-check	1m 4s
mypy (./packages/agbench)	1m 40s
mypy (./packages/autogen-ext)	2m 12s
mypy (./packages/autogen-agentchat)	1m 20s
test (./packages/autogen-core)	1m 1s
mypy (./packages/magentic-one-cli)	1m 23s
test (./packages/autogen-magentic-one)	2m 2s
test (./packages/autogen-ext)	4m 2s
test (./packages/autogen-agentchat)	1m 4s
codecov (./packages/autogen-core)	13s
codecov (./packages/autogen-magentic-one)	13s
codecov (./packages/autogen-ext)	13s
codecov (./packages/autogen-agentchat)	12s
	39m 36s

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