Get-SiteStructure: add new sample and documentation

[tmaestrini]

This contribution adds a new sample to the repository:

The Get-SiteStructure function is a PowerShell function that could be used as a part of a larger script or module. Its purpose is to provide a function that retrieves an existing structure of a SharePoint Online (SPO) tenant and its content for further processing.

The sample adresses SPO admins that are interested in getting a quick and reliable overview over their existing sites (and site content such as document libraries and lists) in the whole SPO tenant (or in a dedicated hub).