Migrate from webpack to turbopack

[ifraixedes]

Next 16 requires Turbopack. We use Webpack, so we have to migrated.

This showed up on #637 by the github action that build a temporary site to verify that changes work

- Experiments (use with caution):
  ✓ scrollRestoration

  Creating an optimized production build ...
[baseline-browser-mapping] The data in this module is over two months old.  To ensure accurate Baseline data, please update: `npm i baseline-browser-mapping@latest -D`
⨯ ERROR: This build is using Turbopack, with a `webpack` config and no `turbopack` config.
   This may be a mistake.

   As of Next.js 16 Turbopack is enabled by default and
   custom webpack configurations may need to be migrated to Turbopack.

   NOTE: your `webpack` config may have been added by a configuration plugin.

   To configure Turbopack, see https://nextjs.org/docs/app/api-reference/next-config-js/turbopack

   TIP: Many applications work fine under Turbopack with no configuration,
   if that is the case for you, you can silence this error by passing the
   `--turbopack` or `--webpack` flag explicitly or simply setting an 
   empty turbopack config in your Next config file (e.g. `turbopack: ***`).
``

[ifraixedes]

To unblock #637 we pass --webpack flag to Next.js build to silence the error

At this time the migration to a Turbopack is complex because it doesn't
support the same level of loader chaining with plugin-based processors
that webpack does and our current webpack loader works by augmenting the
output of @markdoc/next.js which is the Next.js plugin that processes
the markdown files.

We'll look to migrate when Turbopack matures in this aspect.