use up-to-date sphinx_rtd_theme extension

Currently sphinx_rtd_theme is a fixed asset in the repository.

This can lead to various problems:

- search doesn't work anymore when building the doc with recent sphinx
- generated html & js code not in line with current standard
- no fixes from upstream are integrated

BTW, as per the doc at:
  https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html#via-git-or-download

  Installing directly from the repository source is deprecated and is not
  recommended. Static assets won’t be included in the repository starting in
  release 3.0.0.

This removes the theme, and updates conf.py to use the now prefered method to
get it as per:
https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html#how-to-install-and-use-the-theme

Author: tenzap