Update Vulkan backend docs using backend template

[GregoryComer]

📚 The doc issue

Update the Vulkan 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 @SS-JIA @manuelcandales @cbilgin

[GregoryComer]

@SS-JIA Hey Stephen, I've updated this task with a more detailed backend template and everything is merged back into master, so it should now be fully actionable. Could you take a look when you have time? It would be nice to have by 0.6 release (it can be picked), but 0.7 would be fine as well. Also, if you have any concerns with the doc approach, feel free to raise them. Thanks!