Updating actor serialization documentation (dapr#1371)

* Changed headers, updated introduction to reflect the difference in serialization between either type and added a brief section to detail the use of System.Text.Json for weakly-typed Dapr actor clients and to point to official documentation on it

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Signed-off-by: Whit Waldo <[email protected]>
Signed-off-by: Divya Perumal <[email protected]>

Author: WhitWaldo