Pull iOS docs up into the top-level getting started flow

[GregoryComer]

📚 The doc issue

Currently, we have iOS docs under Integrating and Running ExecuTorch on Apple Platforms and some under delegates and such. I'd like to pull the key info up to the top of the documentation, maybe under a "Using ExecuTorch on iOS" page or similar. I'll defer to the iOS experts on content (@shoumikhin), but I'm tentatively thinking we should cover package management and build (the current contents of the above link and a high-level description of the Core ML (and maybe MPS?) delegates with links to additional delegate-specific documentation.

The goal is to provide a top-level "one-stop" page for iOS devs that gives them enough information to get started and integrate, and then guide them top advanced info for more advanced use cases.

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw

[mergennachin]

Subtask of #7903

[mergennachin]

#8511