Document "Dropping Messages" concept

[th0ger]

In SDK implementations and event time filter examples there exists MessageToDrop(...) (go) and to_drop(...) (python).

It is not obvious why dropping messages must be done by adding dropped message with timestamps. (I have my guesses though.)

Please clarify in core numaflow docs the overall concept of:

• why messages dropped in a particular way in stream processing
• how it works/implemented in numaflow,
• in which use cases it is relevant (other than sourcetransformer?)
• if it's required or optional.

I'll be happy to review a doc PR to check if it makes sense.

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