Add user installation/upgrade instructions

[ElliottKasoar]

As requested by @alinelena and others, we may want some example scripts/tutorials describing how to upgrade MACE, PyTorch, ASE (e.g. https://gitlab.com/drFaustroll/ase.git@npt_triangular), etc, as sometimes users require the latest features that are not yet available in published releases.

We may also want to consider user uv instructions, similar to #396, e.g. uv pip install git+https://github.com/stfc/janus-core.git. While most users don't need all of the details added in that PR, it is very fast, and can be useful for installing Python, particularly on HPC systems, similar to micromamba, conda etc.