Introduce and Document Nested Workspaces

     Workspaces should be easy to nest inside each other. Some examples of when this could be useful include when you have:

• A master portfolio in one Git repository and want to include various different projects in it as Git submodules (or subtrees.)
• More than one self-contained sub-unit:
  • Whose files you don't want to let leak out into your repository's top-level root.
  • Whose parent directory you want to sort into a folder outside of 'All Content (Unprocessed)' as a non-decomposable unit, handling it as if it were a single file.

I need to:

1. Finalize how I want this to work.
2. Make sure there isn't anything in my existing documentation that I'd need to rework even slightly to make room for notes on workspace nesting in it.
3. Add the relevant documentation.