quick install guide clarification

[ragusa]

the quick install guide states this at its beginning"

The following packages are required for OpenSn installation and development:

- A recent version of clang++/g++ that supports C++20
- flex (required by the PTSCOTCH component of PETSc)
- Python3 v3.9+ and pip (required by PETSc and OpenSn)
- Git version control system
- CMake v3.20.2+
- MPI (OpenMPI, MPICH, and MVAPICH have been tested)
- PETSc 3.17.0+, Boost 1.86+, HDF5 1.14+, VTK 9.3.0+, Caliper 2.10+
- pybind11 installed via pip
- Pandoc and Doxygen (required for generating the OpenSn documentation)

Can you be more explcit on what the system needs to have before starting the install process, and what the install process will install if not found. Basically, it feels like that the above list should be split into:

• you must install this before starting
• if you do not have those, they will be installed by cmake or by setup.py (pip install) for you; like an advanced notice of what's about to happen. Maybe we should also recommend then that they user creates a virtual env before starting (remember, those instructions are also for neophytes)