add table of contents for exercise book

src/compound-types/array.md

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+# array

src/compound-types/enum.md

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+# enum

src/compound-types/intro.md

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+# Compound Types

src/compound-types/string-slice.md

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+# string and slice

src/compound-types/struct.md

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+# struct

src/compound-types/tuple.md

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+# tuple