Restructure and update installation instructions

[thehrh]

Main changes:

• update and move quick start instructions back to top
• default method suggests using Miniforge (with either conda or mamba as package manager) instead of Anaconda or Miniconda with conda (see e.g. https://www.rc.virginia.edu/2024/10/transition-from-anaconda-to-miniforge-october-15-2024/, https://www.anaconda.com/blog/update-on-anacondas-terms-of-service-for-academia-and-research, https://docs.anaconda.com/anaconda-anon-usage/)
• restructure such that it becomes more apparent which steps are universal; in particular, only clone PISA after setting up environment, so that git may be obtained from conda-forge in the case of installation using the Miniforge distribution
• clean up notes on dependencies
• remove instructions on how to interface with fridge or specific paths on Cobalts (more suitable place for this would be instructions in the fridge repo itself IMO)

[JanWeldert]

I will merge it and we can do the fine tuning in https://github.com/icecube/pisa/tree/Update_install_instructions