build: upgrade to Docusaurus 3

[erickzhao]

At a high level, this PR is making a bunch of breaking changes:

• Docusaurus 3
• MDX 3
• React 18

In order to accomplish that, we had to:

• Change imports across the board from CJS to ESM.
• Convert docusaurus.config.js to docusaurus.config.ts (drops ts-node as a dep).
• Rewrite Remark plugins to use MDX 2+ AST syntax.