feat: integrate i18n router

[watilde]

Related issue

#2648

Description of changes

This PR is to integrate i18n router as a foundation of further i18n integration and should have no breaking change. There seem to have two different type of documentation in terms of translation system - 1) html files with content json 2) mdx files; The scope of this PR is for the first one and I will make PR for automated mdx converting once we align on this router.

i18n router has two features - 1) Use en as a default locale and change nothing with the current website 2) Use local as a prefix for path and router automatically handle routing. Nothing happens if there is no translated resources.

Screenshots

Use en as a default local

https://localhost:3000/

Use ja as a prefix for path

https://localhost:3000/ja/

Use en content when translation does not exist

https://localhost:3000/ja/cli/

[watilde]

CI issue is related to vercel/next.js#18318. I'm investigating to fix now.

[watilde]

Turned out next export does not support i18n due to this reason: i18next/next-i18next#1466 (comment). This seems to be reasonable and we might can start discussion what to do first.

[watilde]

Rebased

[mauerbac]

Will close in favor of team implementing i18n support