Add placeholder runtime/COMPATIBILITY.md (#6307)

Summary:
This file will contain the details of the PTE-to-runtime compatibility policy.

Update the appropriate docs to point to this file. Note that even release branches should point to the `main` branch version of this file, because the latest version is always the official policy.

Pull Request resolved: https://github.com/pytorch/executorch/pull/6307

Test Plan:
Checked links:
- https://docs-preview.pytorch.org/pytorch/executorch/6307/index.html
- https://docs-preview.pytorch.org/pytorch/executorch/6307/pte-file-format.html

Reviewed By: mergennachin

Differential Revision: D64501085

Pulled By: dbort

fbshipit-source-id: ac59529aa12fa76880a3531023a1a308f565ba50