Document: Allow subfolders for Google Drive mounts

[mmattel]

What Needs to be Documented?

A new functionality for Google Drive mounts has been merged in core.
You can now mount a subfolder for GD.
owncloud/core#38161 (Allow subfolders for Google Drive mounts)

Where Does This Need To Be Documented?

https://github.com/owncloud/docs/blob/master/modules/admin_manual/pages/configuration/files/external_storage/google.adoc
(Images, description)

Why Should This Change Be Made? (Optional)

New functionality added

What Type Of Content Change Is This? (Optional)

• New Content Addition
• Old Content Deprecation
• Existing Content Simplification
• Bug Fix to Existing Content

Which Manual Does This Relate To? (Optional)

• Admin Manual
• Developer Manual
• User Manual
• Android
• iOS
• Branded Clients
• Desktop Client
• Other

NOTE: for the time being, this functionality is in core/master but not part of the 10.6 release.
Any PR created must check for which docs branch it is created and backported to.