Upgrade to Docusaurus v3

[diwakergupta]

Not urgent, just filing for tracking.

Docusaurus v3 was released last month and v2 is now in maintenance mode. They recommend upgrading to v3 by January 2024.

The upgrade guide seems mostly straight-forward.

I started making the changes just to get a sense of scope. Right now the primary blocker is the openapi plugin, which does have an approved (though not yet merged) PR for v3 support.

Once that is merged, hopefully this should be an easy lift.

[diwakergupta]

Quick update here: docusaurus has announced v2.x is no longer supported, released 3.0.1. The openapi plugin has also been updated, so that's no longer a blocker.

At this point, the only blocker is actual Markdown changes that break because of the switch from MDX1 to MDX3. This is in line with what they say: https://docusaurus.io/docs/migration/v3

The main breaking change is the upgrade from MDX v1 to MDX v3

Specifically, a bunch of stuff in _commands.md now breaks due to the problems described here: https://docusaurus.io/docs/migration/v3#common-mdx-problems

Basically, bad usages of { and < that now need to be escaped properly.

Addressing this properly likely requires an update to stacks.js as well, I'll see if I can generate a PR easily. FYI @janniks