update README to reflect V5 as current interface

[thedavidmeister]

Summary

V5 is the live interface; V1, V2, V3, and V4 are all under src/interface/deprecated/. The previous README still described V4 as the unstable current target, omitted V5 entirely, and listed only V1+V2 under "Deprecated versions".

Also drops the dead docs URL (https://rainprotocol.github.io/rain.flow returns 404 from GitHub Pages). A working docs site can replace it when one exists.

Closes #352 #354.

Test plan

• forge build clean
• rainix-sol-legal — REUSE compliant

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: ba4cc6e6-c0fe-48e9-b7e3-cba4ff4bec00

📥 Commits

Reviewing files that changed from the base of the PR and between ceaea3c and 6163112.

📒 Files selected for processing (1)

• README.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-05-issue-352-354-readme-update

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