Stop using title frontmatter and fix doc that can only be reached by search (#20623)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>

docs/contributing/model/README.md

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-title: Summary
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+# Summary
 
 !!! important
     Many decoder language models can now be automatically loaded using the [Transformers backend][transformers-backend] without having to implement them in vLLM. See if `vllm serve <model>` works first!

docs/contributing/model/basic.md

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-title: Basic Model
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+# Basic Model
 
 This guide walks you through the steps to implement a basic vLLM model.
 

docs/contributing/model/multimodal.md

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-title: Multi-Modal Support
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+# Multi-Modal Support
 
 This document walks you through the steps to extend a basic model so that it accepts [multi-modal inputs](../../features/multimodal_inputs.md).
 

docs/contributing/model/registration.md

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-title: Registering a Model
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+# Registering a Model
 
 vLLM relies on a model registry to determine how to run each model.
 A list of pre-registered architectures can be found [here](../../models/supported_models.md).

docs/contributing/model/tests.md

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-title: Unit Testing
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+# Unit Testing
 
 This page explains how to write unit tests to verify the implementation of your model.