Update package Docusaurus to latest version (fix compatibility with node 17+)

[flodolo]

I've tried to run the website locally, and with node v18 it doesn't work. The only way to run it is to force legacy SSL provider, i.e. NODE_OPTIONS=--openssl-legacy-provider yarn start. See also this post.

So I looked at the dependencies, and Docusaurus is using a strange version (2.0.0-beta6, while the latest is 2.4.1).

With the latest version, everything works normally, but the update is not straightforward:

• There is a config change for hideable sidebar (easy).
• It requires a migration to a different search system (details here).
• sidebar.js seems broken (because of Early Startup and Unenrollment). For what it's worth, I think I've seen warnings displayed in console even with the current version, but there it builds.

P.S. I didn't test it in depth, there might be other things broken that don't show up in the build phase.

[flodolo]

The error in the current version is

No docs found in dir faq/early-startup: can't auto-generate a sidebar.
No docs found in dir faq/unenrollment: can't auto-generate a sidebar.

Which I assume was introduced in #418

[flodolo]

The error in the current version is

This was fixed in #444