Move legacy FEniCS documentation to /documentation/archive and add links to FEniCSx documentation
Move legacy FEniCS installation instructions to /download/archive and add details of some methods of installing FEniCSx (see #71 for others that still need adding)
Update info on community page
Add a FEniCSx vs legacy FEniCS section to the front page.
Closes #27. Closes #35. Closes #68.