Update installation documentation

I've just published xsocs in pypi: https://pypi.org/project/xsocs/ which makes installation simpler and ensure storing older versions more than in gitlab package registry: For version 2020.11.0, the tarball and wheels are referenced but the files are no longer available... (https://gitlab.esrf.fr/kmap/xsocs/-/packages/1).

Documentation needs to be updated accordingly.