docs: update AssistantAgent documentation with a new figure, attention and warning notes #3609

Job	Run time
lint	1m 13s
docs (./packages/autogen-core)	1m 40s
docs-example-check (./packages/autogen-core)	2m 56s
format	52s
markdown-code-lint	2m 1s
mypy (./packages/autogen-core)	2m 33s
pyright (./packages/autogen-core)	58s
mypy (./packages/autogen-magentic-one)	1m 38s
pyright (./packages/autogen-magentic-one)	1m 2s
samples-code-check	1m 7s
mypy (./packages/agbench)	1m 42s
pyright (./packages/agbench)	1m 13s
mypy (./packages/autogen-ext)	2m 13s
mypy (./packages/autogen-agentchat)	1m 31s
test (./packages/autogen-core)	1m 14s
pyright (./packages/autogen-ext)	1m 15s
mypy (./packages/magentic-one-cli)	2m 44s
test (./packages/autogen-magentic-one)	2m 12s
pyright (./packages/autogen-agentchat)	1m 34s
test (./packages/autogen-ext)	3m 49s
pyright (./packages/magentic-one-cli)	1m 3s
test (./packages/autogen-agentchat)	1m 8s
check-proto-changes-python	1m 8s
codecov (./packages/autogen-core)	10s
codecov (./packages/autogen-magentic-one)	9s
codecov (./packages/autogen-ext)	8s
codecov (./packages/autogen-agentchat)	10s
	39m 23s

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