Update Cadence Xtensa backend docs using backend template

[GregoryComer]

📚 The doc issue

Note: This issue pertains to the executorch-just-works branch. It is intended to merge back into master within the next 1-2 weeks.

Update the Cadence Xtensa backend top-level documentation page (docs/source/backends-cadence.md) to use the new backend template (docs/source/backend-template.md). See the 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.

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