docs: add data packing and serialization guide

[critesjosh]

Summary

• Adds a new documentation page explaining the difference between Serialize/Deserialize and Packable traits in Aztec.nr
• Covers when each trait is used, how to write custom Packable implementations for gas optimization, and common pitfalls
• Includes a worked example contract (packing_example) demonstrating manual packing
• Cross-references the new page from existing docs (custom notes, events, macros, globals) and adjusts sidebar positions

Test plan

• Verify yarn build passes in docs/
• Verify spellcheck passes (yarn spellcheck)
• Confirm the new page renders correctly and sidebar ordering is correct
• Verify packing_example contract compiles with nargo compile
• Check all cross-references link correctly