DOCS: document an easier method of plausible analytics (#1867)

- More direct injection of plausible.io analytics, using nothing but
  a standard Sphinx option.
- I have tested this in the jupyter-book docs.  This solution uses `<script
  defer="defer" ...>` instead of `<script defer ...>`, but the
  plausible people say it should be the same.  The advantage here is
  that the configuration is more simple and stardard.
- Apparently this will also be possible by the pydata sphinx theme,
  it's up to you all if you prefer theme-based or standard config
  based.  Feel free to close this PR if you would rather sphinx-based.