Update mps docs (#974)

Summary:
Summary of changes:
- Update MPS docs to include Xcode15 as prerequisite software
- Fix anchor links for executorch tutorial

Pull Request resolved: https://github.com/pytorch/executorch/pull/974

Reviewed By: cccclai

Differential Revision: D50347558

Pulled By: shoumikhin

fbshipit-source-id: 1950dfbbcce179cb0e3c0f820f8cb5eafc39c34c