Basic metadata for SEO

A couple of easy changes to make:

• improve the description on the pages
• generate the sitemap https://nextjs.org/docs/app/api-reference/file-conventions/metadata/sitemap
• robots.txt

The root https://api.grid5000.fr/explorer/ has a 401 somewhere in the page to automatically authenticate the user, while the public-api has not. I'm unsure how well this will be treated by a robot, maybe we should require an action from the user for the authentication?