doc: add package-level doc comments (#9)

examples/add/doc.go

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+// Package add demonstrates an Add function in avo.
+package add

examples/args/doc.go

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+// Package args demonstrates how to load function arguments in avo.
+package args

examples/complex/doc.go

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+// Package complex shows how to work with complex types in avo.
+package complex

examples/data/doc.go

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+// Package data shows how to define DATA sections in avo.
+package data

examples/dot/doc.go

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+// Package dot implements vector dot product in avo.
+package dot

examples/fnv1a/doc.go

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+// Package fnv1a implements FNV-1a in avo.
+package fnv1a

examples/geohash/doc.go

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+// Package geohash implements integer geohash encoding in avo.
+package geohash

examples/returns/doc.go

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+// Package returns demonstrates how to write function return values in avo.
+package returns

examples/sha1/doc.go

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+// Package sha1 implements the SHA-1 hash in avo.
+package sha1

examples/sum/doc.go

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+// Package sum sums a slice in avo.
+package sum