Add FEniCSx installation instructions

[mscroggs]

• Add instructions for using FEniCSx with Docker #68
• Installing from source
• Installing with conda
• Installing with pip
• Installing with spack
• Check that this list of installation methods is complete or add to it

[mscroggs]

This issue should maybe be moved to DOLFINx, and the instructions placed at https://docs.fenicsproject.org/dolfinx/main/python/installation.html