Update Registry App landing page to divert data users to separate landing page for using the PDS API

[jordanpadams]

Per inputs from PDS API WG, if a data user comes across https://nasa-pds.github.io/pds-registry-app/, they may be excited to use the API, but no idea how to use it. Let's include a little "Info" box, that says "For Data Users, see PDS API Client User Guide and Documentation here." with link to https://nasa-pds.github.io/pds-api-client/

Some ideas for things we could do:

• Add "Info" box or bold NOTE: somewhere on the page, that says "For Data Users, see PDS API Client User Guide and Documentation here."

Other ideas welcome.

relates to NASA-PDS/pds-api#158 to update API Client documentation