[Local GC] Add a Standalone GC loader design document (#14435)

[Local GC] Add a Standalone GC loader design document (#14435)

* Add a Standalone GC loader design document

* First round of feedback:
        1. Remove some stuff from the preamble that doesn't belong in a
        design document
        2. Simplify the three-variable loading approach to just use one
        variable that contains the path of a GC to load.
        3. Clean up the wording in a few places.
        4. Remove the "state machine" section and reference to state
        machines in general.