Update MPS backend docs using backend template

[GregoryComer]

📚 The doc issue

Update the MPS backend top-level documentation page (docs/source/backends-coreml.md) to use the new backend template (docs/source/backend-template.md). See backend-xnnpack.md for an example. Feel free to deviate or suggest changes to the template if it does not make sense for this backend.

The template (docs/source/backend-template.md) includes the following information:

• Delegate features
• Development requirements
• Target requirements
• Partitioner API
• Quantization
• Runtime integration

General documentation guidelines that I'm hoping to follow:

• Leverage brief in-line code samples, where possible
• Keep things concise
• Avoid first person (I/we)
• I'm generally trying to follow https://developers.google.com/style/highlights

Suggest a potential alternative/fix

No response

cc @mergennachin @byjlw

[GregoryComer]

@cccclai I've updated the issue description and backend template with additional details, and this should be fully actionable now. Could you help in updating the docs or finding the right POC? Thanks!