feat: handle locale autodetection in Netlify redirects

[ascorbic]

We should implement Next.js locale handling using Netlify redirects. If there are multiple locales and auto-detection is enabled, we should add the following redirects:

• If browser language does not match the default, but does match one of the other locales, then redirect all requests to / to the appropriate lcoale prefix.
• If NEXT_LOCALE cookie is set, then rewrite to /.netlify/functions/___netlify-handler instead (to skip the locale detection)
• Add docs, suggesting that they use the nf_lang cookie instead of NEXT_LANG, for better performance.