Docs: Restructure README and split guides into docs/en + docs/ja

[ryz310]

What

• top-level README.md and README.jp.md were shortened into quick-start hub pages
• detailed documentation was split into chapter files under docs/en and docs/ja
• EN/JA docs were aligned to the same chapter structure

Why

• the previous README files were too long to scan quickly
• moving details into chapter files improves discoverability and maintenance
• mirrored EN/JA structure makes updates easier and reduces drift

Included Changes

• rewrote README.md to: overview, install, quick start, doc links
• rewrote README.jp.md to the same concise structure
• added docs/en/01-06 chapters:
  • getting started
  • usage and pagination
  • error handling
  • retry / timeout / logger
  • RSpec helpers
  • development and release
• added matching docs/ja/01-06 chapters
• fixed README typos and example inconsistencies found during the split

Scope

• docs only
• no runtime behavior changes

Verification

• manual link/path check for all docs listed from README files
• no tests run (documentation-only change)

[ryz310]

Recreate on #1180