docs: update AssistantAgent documentation with a new figure, attention and warning notes #3603

Job	Run time
check-proto-changes-python	1m 18s
docs (./packages/autogen-core)	1m 38s
docs-example-check (./packages/autogen-core)	3m 2s
lint	51s
format	49s
markdown-code-lint	2m 12s
mypy (./packages/autogen-core)	2m 25s
pyright (./packages/autogen-core)	56s
mypy (./packages/autogen-magentic-one)	1m 31s
pyright (./packages/autogen-magentic-one)	58s
mypy (./packages/agbench)	56s
samples-code-check	57s
mypy (./packages/autogen-ext)	2m 11s
pyright (./packages/agbench)	54s
mypy (./packages/autogen-agentchat)	1m 27s
pyright (./packages/autogen-ext)	1m 4s
mypy (./packages/magentic-one-cli)	1m 35s
pyright (./packages/autogen-agentchat)	1m 7s
test (./packages/autogen-core)	59s
pyright (./packages/magentic-one-cli)	1m 10s
test (./packages/autogen-magentic-one)	2m 5s
test (./packages/autogen-ext)	25m 8s
test (./packages/autogen-agentchat)	1m 5s
codecov	0s
	56m 18s

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