landing page registry docs to be updated for new registry struct

[yangcao77]

/area documentation

Which area this documentation change is related to?

/area registry

Issue details

What mistake did you find / what is missing in the documentation?
Since #735 has been done, landing page docs need to be updated for changes in registry.

Changing need to be made in
https://devfile.io/docs/devfile/2.2.0/user-guide/building-a-custom-devfile-registry/
https://devfile.io/docs/devfile/2.2.0/user-guide/adding-a-registry-schema/
https://devfile.io/docs/devfile/2.2.0/user-guide/referring-to-a-parent-devfile-in-a-devfile/

[yangcao77]

pr created for landing page: devfile/docs#154
rest API doc changes: johnmcollier/registry-docs#4

[yangcao77]

The landing page change has been merged, closing this PR