Improve linkages from Registry App Docs to API Docs

[jordanpadams]

• overall improvements to documentation from registry to API for new users
• update client docs with links to jupyter notebooks for examples
• update client docs with improved title casing
• stopgap improvement Update Registry App landing page to divert data users to separate landing page for using the PDS API nasa-pds-engineering-node/pds-registry-app#158

[tloubrieu-jpl]

For "overall improvements to documentation from registry to API for new users" see note on https://nasa-pds.github.io/registry/

I am not linking the jupyter notebook yet because we don't have an API server yet which contains the data the notebooks have been developed for. A new ticket is created for that #178