Update to gentype v4 docs

[ryyppy]

@cristianoc If you want to run this project locally, make sure you use node@12 or higher and npm@7.

npm i
npm run update-index
npx rescript

npm run dev
# open localhost:3000

Sidebar is defined in ./data/sidebar_gentype_latest.json, which points to each existing file within pages/docs/gentype/latest.