docs: add a Docs section to AGENTS.md - packages/cddl/docs needs manual sync

[dprevost-LMI]

packages/cddl/docs/*.md hand-documents the AST with worked examples and isn't generated from ast.ts/parser.ts, so nothing keeps it in sync automatically. Found stale twice in this session alone (IsFloat missing from references.md/ranges.md; two whole operators, the OperatorType union, and worked examples missing from operators.md) - both only caught by manual review. Write it down so it stops being a surprise.

[greptile-apps]

Confidence Score: 5/5

The PR appears safe to merge as written.

Summary

Adds contributor guidance explaining that the CDDL AST documentation is manually maintained and must be reviewed whenever AST structures change.

• Identifies packages/cddl/docs/ as the relevant documentation area.
• Calls out worked examples and enumerated lists as synchronization points.
• Records previous documentation drift as rationale for manual review.

_{Reviews (1) · Last reviewed commit: "docs: add a Docs section to AGENTS.md - ..."}