Update autogen_core.memory: adding docs, removing some fields. #3473

Job	Run time
lint	48s
mypy (./packages/autogen-core)	2m 3s
format	1m 0s
mypy (./packages/autogen-magentic-one)	1m 17s
pyright (./packages/autogen-core)	1m 11s
mypy (./packages/agbench)	55s
mypy (./packages/autogen-ext)	2m 6s
pyright (./packages/autogen-magentic-one)	1m 8s
test (./packages/autogen-core)	56s
pyright (./packages/agbench)	57s
mypy (./packages/autogen-agentchat)	1m 8s
test (./packages/autogen-magentic-one)	1m 14s
pyright (./packages/autogen-ext)	1m 11s
mypy (./packages/magentic-one-cli)	1m 11s
test (./packages/autogen-ext)	1m 14s
pyright (./packages/autogen-agentchat)	1m 5s
test (./packages/autogen-agentchat)	59s
pyright (./packages/magentic-one-cli)	1m 0s
check-proto-changes-python	1m 12s
codecov (./packages/autogen-core)	1m 3s
docs-example-check (./packages/autogen-core)	2m 41s
codecov (./packages/autogen-magentic-one)	1m 22s
docs (./packages/autogen-core)	1m 40s
codecov (./packages/autogen-ext)	1m 21s
samples-code-check	56s
codecov (./packages/autogen-agentchat)	1m 6s
markdown-code-lint	2m 12s
	34m 56s

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