Improve Globus guest collection docs

[b-reyes]

To improve our guest collections Guest Collections (Globus Shared Endpoints) documentation, we should incorporate another paragraph explaining what these can do (allow for read/write access, set permissions etc.) and why you might want to set up a guest collection. Additionally, we should consider adding a link / blurb about CU Scholar for long-term storage, as it is a common question related to PL and data sharing.