Update docusaurus to v3+

[pepopowitz]

Must follow #2719.

See #4799 for work breakdown and progress.

Implementation notes

• This might be a nice forcing function to permanently introduce generation of older API docs (which would have been nice for something like Update operate-openapi.yaml #4736).
  • The biggest challenge with this is that the api doc generator emits a typescript sidebar schema file, but the versioned docs require one flat JSON sidebar schema at the version root.
  • But it might not be too bad to hack, because the generated typescript sidebar schema merely exports a single JSON document, and doesn't have any actual javascript functionality.

[pepopowitz]

A note to inspire a little more urgency/relevance for this issue:

I am working on #3649, and wanted to tackle https://github.com/camunda/developer-experience/issues/38 while there, but couldn't because of our older version of docusaurus.

Basically, I wanted to utilize markdown imports to extract a shared authentication document, but because of this issue, I would have had to write the entire document as html instead of markdown. I then looked into updating to mdx v2 via a theme for docusaurus v2, but even that had issues that would have caused me to rewrite a ton of unrelated content.

If we'd been updated to docusaurus v3, theoretically, my markdown imports should have worked without unrelated changes.