Add a root documentation page that links to user and deployment guides

[elookpotts-nvidia]

GitHub repos can only have a single documentation link in the repo description. We have two guides - user and deployment - that we'd like to link to.

This issue tracks making a root page that links to the other guides.

Acceptance criteria

• Root page at https://nvidia.github.io/OSMO
• Looks like what's next
• Has cards for user and deployment guides