Code structure: Clarify VirtualBuffers vs NVDAObjects

[feerrenrut]

Is your feature request related to a problem? Please describe.

As a developer understanding the structure and differences between classes in the virtualBuffers folder and the NVDAObjects/IAccessible/ folders is important. At the moment there seems to be some confusion about the locations of these clasess

Describe the solution you'd like

• Add docs to help developers decide if the code they are adding belongs in one location or the other.
• Move classes for virtualBuffers to new or existing modules in the virtualBuffers folder.

Describe alternatives you've considered

• Leave the code as is

Additional context

• Moving code will result in deprecations, and may not be possible in a backwards compatible way.

[feerrenrut]

This could potentially be done without breaking the API by importing the moved module into the old package. This wouldn't hurt developer comprehension too much if it is enclosed in a deprecation condition and comment to explain the moved code.