First attempt at a document on idempotency

[oggy-dfin]

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[derlerd-dfinity]

Thank you. LGTM.

[robin-kunzler]

Looks great @oggy-dfin , thanks a lot for putting in the work!

Really like the explicit listing of advantages and disadvantages for the sequence numbers and deduplication approaches. This makes it much clearer.

Approving, left a few mostly minor comments / questions below.