Remove `conf.py` and replace contents by config manipulation

[astrojuanlu]

When I created the theme, I was under the impression that the theme conf.py would serve as a baseline that then was automatically extended by the site conf.py.

However, after debugging with @Huongg, I got the feeling that the site conf.py was overriding the theme conf.py.

And finally, after a quick investigation, I realized that normally Sphinx theme don't have their own conf.py.

See for example:

• sphinx-rtd-theme does not have a conf.py https://github.com/readthedocs/sphinx_rtd_theme/
• neither does furo https://github.com/pradyunsg/furo/
• nor pydata-sphinx-theme, although this one has an update_config function connected to the builder-inited hook that reads app.config.extensions value https://github.com/pydata/pydata-sphinx-theme/blob/11e041/src/pydata_sphinx_theme/__init__.py

We'd need to look into this closely.