[ET-VK][docs] Update to the new template

[SS-JIA]

Summary:
Title says it all! Add Vulkan docs in accordance to the format for XNNPACK introduced in #14873.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/14996

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❌ 15 Cancelled Jobs, 1 Unrelated Failure

As of commit 2d9e333 with merge base 4ba2a66 ():

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FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / unittest-arm-backend-with-no-fvp (test_pytest_models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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[digantdesai]

docs are not rendered :(