Update Cadence Xtensa backend docs using backend template

[GregoryComer]

📚 The doc issue

Update the Cadence Xtensa 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 @mcremon-meta