Is there a limit on how many folders/sub-folders/sub-sub-folders etc that we can have?
No, there are no limits on the number of folders or how deep you want to nest them.
Is there a search function for the Governance Documents?
As of Aug 2017 there is no search function, but that is scheduled for release in Sept 2017.
What are restricted folders?
Restricted folders, also known as Board Only folders, are folders for board members only. Not even administrators can access a restricted folder (unless they also happen to be a board member).