add table of contents for exercise book

src/smart-pointers/box.md

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+# Box

src/smart-pointers/cell-refcell.md

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+# Cell and RefCell

src/smart-pointers/deref.md

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+# Deref

src/smart-pointers/drop.md

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+# Drop

src/smart-pointers/intro.md

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+# Smart pointers

src/smart-pointers/rc-arc.md

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+# Rc and Arc