docs: update README for clarity and conciseness, add migration guide (#28)

* docs: update README for clarity and conciseness, add migration guide for goerr v2, and enhance feature descriptions to improve user understanding

* docs(README): remove Go Reference badge to streamline the documentation

* docs(README): update project description to clarify enhanced error handling features

* docs(migration_v2): update migration guide to include details on replacing errors.Cause() with a loop for root cause retrieval

* docs: update migration guide link in README and add detailed migration guide for goerr v2

* fix(docs): update README to reflect changes in processFile function by removing unused variable data

Author: m-mizutani