add table of contents for exercise book

src/threads/atomic.md

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+# Atomic

src/threads/basic-using.md

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+# Basic using

src/threads/intro.md

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+# Threads

src/threads/message-passing.md

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+# Message passing

src/threads/send-sync.md

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+# Send and Sync

src/threads/sync.md

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+# Sync