[Infra] Migrate/update documentation

The current docs dummy app is showing its age. It still depends on TED header etc.

Options to consider:
- @empress/field-guide
- [docfy](https://docfy.dev/)

Other ideas?

Start by reviewing the current docs dummy app and its TED header dependency, then compare the mentioned @empress/field-guide and docfy options. Done should be a decided migration or update scope with the selected approach documented; the issue does not name files or tests to run.

索引モデルが issue の本文から書いたものです。