Add FEniCSx installation instructions

[mscroggs]

• [x] https://github.com/FEniCS/web/issues/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