[DOC][ROCm]: Add attention backend guide

[vllmellm]

Purpose

This PR adds a new documentation page attention_backends.md under the "Features" section.

It provides a comprehensive guide on:

• The differences between available ROCm attention backends (TRITON_ATTN, ROCM_ATTN, ROCM_AITER_FA, etc.).
• How to use the VLLM_ATTENTION_BACKEND environment variable to select specific backends.
• Backend selection priority logic.
• Configuration for DeepSeek MLA models on ROCm.

This documentation corresponds to the changes and features solidified in PR #26980.

Test Plan

• Built the documentation locally using mkdocs serve.
• Verified the rendering of tables, code blocks, and admonitions.

Test Result

The documentation renders correctly.

Preview:

[mergify]

Documentation preview: https://vllm--29400.org.readthedocs.build/en/29400/