Document DropPoint integration

[shunichironomura]

Summary

• Add a DropPoint integration guide with setup requirements and Procnote environment variables
• Add a blurred QR-code upload screenshot to the docs
• Add a Procnote execution-page screenshot to the README and docs home page, with a note that it is based on procedures/example-tvt/template.md
• Link the new guide from the documentation navigation and execution guide
• Update documentation site metadata in zensical.toml

Checks

• just test
• just check-vp (passes with existing unicorn/no-empty-file warning for src/lib/index.ts)
• zensical build --clean