Add unimodal embedding documentation and other doc clean ups

[sabrinacamp2]

GitHub repository README.md

• Added section on generating spatial fusion inputs (Prepare SpatialFusion inputs), linking out to the mkdocs website documentation (will refresh that branch after this PR is merged, then the link will work)
• cleaned up Installation section now that we have the unimodal embedding generation step documented separately. The Installation section here can just focus on whats needed for spatial fusion package.
• added biorxiv citation

mkdocs

• Added citation to Home page
• Revised Installation page in the same way that the repo README.md installation section was revised
• added new section "Prepare inputs" which documents the two ways we support generating unimodal embeddings