Add mdx-html generator (Legacy Generator)

[ovflowd]

It will take some time since we still need to create all the new API Docs Components and complete the Shiki, React, etc., migration within this repository.

To use the new tooling and ensure that we can test the generation of the API docs with this new system, we should set up a temporary mdx-html generator that generates the same layout as today.

The HTML can be a bit different underneath, but the goal is to generate (code-wise) as close as possible to the existing API docs and a visual match of both. We can even use some tools to compare how different they look/etc.

The generator would probably look as:

generators/
  mdx-html/
    index.mjs
    templates/
      index.html <-- based on https://github.com/nodejs/node/blob/main/doc/template.html
    assets/ <-- imported from https://github.com/nodejs/node/tree/main/doc/api_assets