Fix broken images in docs (#5514)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/5514

https://github.com/pytorch/executorch/pull/5475/ moved the images to static folder, https://github.com/pytorch/executorch/pull/5493 updated some of it but missed out on the delegate specific docs.

This diff, fixes that by replacing the image URLs that are broken

Reviewed By: cmodi-meta, svekars, kirklandsign

Differential Revision: D63135522

fbshipit-source-id: 10071b0428c203eef38500852e64f71e13ec1315