update deps

[thibautsabot]

What have been done :

Updated all deps ! (gatsby v3, sass)
Updated node version
Now using scss instead of sass
Moved some components around to get rid of warnings
hardcoded title and description metadata instead of a graphql query to avoid some dev crashes

TODO :

• [gatsby-plugin-netlify] depracated webpack-assets-manifest causing errors during build gatsbyjs/gatsby#29974
• Fork https://github.com/gatsby-contrib/gatsby-plugin-elasticlunr-search
• [gatsby-transformer-sharp] The "fixed" and "fluid" resolvers are now deprecated. Switch to "gatsby-plugin-image" for better performance and a simpler API. See
  https://gatsby.dev/migrate-images
• Check image compressions (+ avif)
• Global QA (+ responsive)

If we want to use React v17 we need to use --legacy-peer-deps to npm
(check bundle size with react 16 vs 17)