Remove final references to /docs/website/... (#1094)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1094

portable/README.md: Use the same path as a similar link later in the document.

xnnpack/README.md: The URLs were broken; point to the public website, which will render them more nicely.

These are the last references to `docs/website` apart from files that actually live under `docs/website`.

Reviewed By: mergennachin

Differential Revision: D50627786

fbshipit-source-id: 37ec0bf2b85eae6f75a6640c7917459f33607044