Review Explorer package(s) documentation

With changes for NXP-28837, the following will probably need be updated:

• https://connect.nuxeo.com/nuxeo/site/marketplace/package/platform-explorer (apparently it's missing an icon, and maybe the description can be improved)
• https://connect.nuxeo.com/nuxeo/site/marketplace/package/platform-explorer-jsf has been extracted, so maybe it should be mentioned in upgrade notes (and maybe the description can be improved)

Depending on how other changes go for issues in this epic, please let me know if:

• i should create a dedicated ticket for "user" documentation on new features
• i should create a dedicated ticket for "upgrade notes" documentation of compatibility breaks (as some features are being dropped, and code api has changed)