Create documentation landing page

[sdruskat]

• Currently, the "Documentation" link links to the README.
• To accommodate tutorials, create a landing page that will
  • link to the docs (README, guide) at the top
  • Includes tutorials on validation w/ validator.py, cffconvert (when supported), ruby-cff, ideally link to a validator that's easier to use, e.g., a Jupyter Notebook hosted somewhere (GitHub Codespaces, binderhub, etc.), or a simple web app based on jsonschema or similar (see if we can reuse any of the existing linters!)

[sdruskat]

Not sure about this anymore, perhaps we should just add a new nav link to a dedicated Tutorials page.

[sdruskat]

Not sure about this anymore, perhaps we should just add a new nav link to a dedicated Tutorials page.

Done in #34.