[Doc] Update V1 User Guide for Hardware and Models

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.

Purpose

Regarding hardware support:

• V1 is currently enabled by default for TPU ([V1] Enable TPU V1 backend by default #17673), CPU x86 ([CPU] V1 support for the CPU backend #16441, [Bugfix] fix v1 cpu worker fails on macOS #19121), AMD ([ROCm][V1] Adding ROCm to the list of plaforms using V1 by default #19440)

Regarding model support:

• Mamba V1 support is now in WIP stage ([v1] Support mamba2 #19327)

Other changes:

• Reduce nesting of the headers
• Split up models and features into separate sub-sections

Test Plan

N/A

Test Result

N/A

(Optional) Documentation Update

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