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

Summary:
Pull Request resolved: #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

Author: dbort

examples/portable/README.md

@@ -1,6 +1,6 @@
 # ExecuTorch in Portable Mode
 
-This dir contains demos to illustrate an end-to-end workflow of using ExecuTorch in [portable mode](/docs/website/docs/basics/terminology.md).
+This dir contains demos to illustrate an end-to-end workflow of using ExecuTorch in [portable mode](../../docs/source/concepts.md#portable-mode-lean-mode).
 
 
 ## Directory structure

examples/xnnpack/README.md

@@ -1,8 +1,8 @@
 # XNNPACK Backend
 
 [XNNPACK](https://github.com/google/XNNPACK) is a library of optimized neural network operators for ARM and x86 CPU platforms. Our delegate lowers models to run using these highly optimized CPU operators. You can try out lowering and running some example models in the demo. Please refer to the following docs for information on the XNNPACK Delegate
-- [XNNPACK Backend Delegate Overview](https://github.com/pytorch/executorch/blob/main/docs/website/docs/source/native-delegates-executorch-xnnpack-delegate.md)
-- [XNNPACK Delegate Export Tutorial](https://github.com/pytorch/executorch/blob/main/docs/website/docs/source/tutorial-xnnpack-delegate-lowering.md)
+- [XNNPACK Backend Delegate Overview](https://pytorch.org/executorch/stable/native-delegates-executorch-xnnpack-delegate.html)
+- [XNNPACK Delegate Export Tutorial](https://pytorch.org/executorch/stable/tutorial-xnnpack-delegate-lowering.html)
 
 
 ## Directory structure