docs: add migration guide from jq/yq/mlr to morph

[alvinreal]

Summary

Adds docs/MIGRATION.md — a comprehensive side-by-side recipe guide for users migrating from jq, yq, or mlr (Miller) to morph.

Sections covered

• Format conversion — basic format-to-format examples
• Field selection, renaming, dropping — with jq/yq/mlr equivalents
• Filtering rows — single and compound conditions
• Adding/computing fields — static values and computed expressions
• Type casting — int, float, string, bool
• Nested data — flatten and nest operations
• Sorting — ascending and descending
• String operations — lower, upper, replace, join, split, trim, len
• Conditional logic — when blocks
• Array iteration — each blocks
• Defaults/null handling — default and coalesce
• Chaining operations — pipeline composition
• Caveats & differences — comparison table, limitations, common gotchas
• Quick reference card — one-liner summary of all operations

README changes

Added a Documentation section linking to the mapping language reference, installation guide, and migration guide.

Fixes #83