Update ReadTheDocs config to use the new build images

[raucao]

It's now possible to ditch all the custom workarounds and environment and just specify a node.js version as well as npm packages to install, like so:

build:
  os: ubuntu-22.04
  tools:
    python: "3.9"
    nodejs: "16"
  jobs:
    post_install:
      - npm install -g jsdoc

readthedocs/readthedocs.org#3069 (comment)

Hopefully this also fixes the builds being broken again (and more sustainably).