Create a landing page for executorch mobile

[byjlw]

📚 The doc issue

Currently there is no landing page for folks searching for pytorch on mobile and now pytorch mobile redirects to executorch which covers everything

Suggest a potential alternative/fix

Landing page specific to those looking for a pytorch on mobile solution and will link to the specific android and ios documentation as well as some high level details on why you'd use ExecuTorch for mobile

cc @mergennachin

[cbilgin]

Any reason for linking rather than just deleting the original docs?

[cbilgin]

These two seem to be the 2 big ones

• https://pytorch.org/mobile/android/
• https://pytorch.org/mobile/ios/

In PyTorch code

• https://github.com/pytorch/android-demo-app
• https://github.com/pytorch/mobile-docs
• https://github.com/pytorch/tutorials/blob/67667efbc6acdf5f82a9c8be5000801e406edc50/index.rst#L1127

And blog, likely should be deleted

• https://pytorch.org/blog/mobile-demo-apps-overview/
• https://pytorch.org/blog/introducing-the-playtorch-app/

Do we delete discussions too?

• https://discuss.pytorch.org/t/options-for-mobile-inference/176200
• https://discuss.pytorch.org/t/model-training-on-mobile-devices/113733