docs: use new docs.postmarketos.org theme

The documentation at https://docs.postmarketos.org is getting reworked to use a new theme that is from now on stored in one common place: https://gitlab.postmarketos.org/postmarketOS/docs.postmarketos.org/-/tree/main/theme

* Install packages for docs with pip inside a venv, instead of
  installing the packages with apk. This is needed as the custom theme package.
  It also allows building the docs by running "make" in the docs dir
  from now on, in addition to using "pmbootstrap ci docs".
* Use "Options: native" in the CI script, as all packages are now
  getting installed via pip, and this is faster. Storing artifacts from
  the public dir is not needed, as the output doesn't need to be grabbed
  from a chroot, it is directly in the git repository.
* Adjust the config for the new theme.

Other commits tweak the docs and adjust the CI trigger.