[#105] *: Add doc.go files for each contract

Signed-off-by: Alex Vanin <alexey@nspcc.ru>

alphabet/doc.go

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+// Alphabet contract description.
+package alphabet

audit/doc.go

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+// Audit contract description.
+package audit

balance/doc.go

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+// Balance contract description.
+package balance

container/doc.go

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+// Container contract description.
+package container

neofs/doc.go

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+// NeoFS contract description.
+package neofs

neofsid/doc.go

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+// NeoFSID contract description.
+package neofsid

netmap/doc.go

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+// Netmap contract description.
+package netmap

processing/doc.go

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+// Processing contract description.
+package processing

proxy/doc.go

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+// Proxy contract description.
+package proxy

reputation/doc.go

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+// Reputaiton contract description.
+package reputation