docs: Musicgen melody model card
Author: AshAnand34
Pull request: yes

# What does this PR do?

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This pull request updates the documentation for the MusicGen Melody model in `docs/source/en/model_doc/musicgen_melody.md`. The changes aim to simplify and enhance the clarity of the documentation by restructuring the content, adding examples, and improving formatting.

### Documentation Improvements

#### Overview and Structure
* Reorganized the content to provide a concise overview of the MusicGen Melody model, highlighting its key features and differences from the original MusicGen.
* Simplified the explanation of the model's architecture, breaking it down into three main components: text encoder, MusicGen Melody decoder, and audio decoder.

#### Examples and Usage
* Replaced lengthy code snippets with streamlined examples for generating music using text and audio prompts, including text-only and unconditional generation scenarios.
* Added detailed examples for using tools like Demucs for melody isolation and quantization techniques for memory optimization.

#### Formatting and Accessibility
* Introduced a sidebar navigation and collapsible sections for better readability and user experience.
* Updated links to external resources and added tooltips for key concepts like quantization and guidance scale.

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#36979


## Before submitting
- [x] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@stevhliu 

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Models:

- text models: @ArthurZucker
- vision models: @amyeroberts, @qubvel
- speech models: @eustlb
- graph models: @clefourrier

Library:

- flax: @gante and @Rocketknight1
- generate: @zucchini-nlp (visual-language models) or @gante (all others)
- pipelines: @Rocketknight1
- tensorflow: @gante and @Rocketknight1
- tokenizers: @ArthurZucker
- trainer: @zach-huggingface, @SunMarc and @qgallouedec
- chat templates: @Rocketknight1

Integrations:

- deepspeed: HF Trainer/Accelerate: @SunMarc @zach-huggingface
- ray/raytune: @richardliaw, @amogkam
- Big Model Inference: @SunMarc
- quantization (bitsandbytes, autogpt): @SunMarc @MekkCyber

Documentation: @stevhliu

HF projects:

- accelerate: [different repo](https://github.com/huggingface/accelerate)
- datasets: [different repo](https://github.com/huggingface/datasets)
- diffusers: [different repo](https://github.com/huggingface/diffusers)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Maintained examples (not research project or legacy):

- Flax: @Rocketknight1
- PyTorch: See Models above and tag the person corresponding to the modality of the example.
- TensorFlow: @Rocketknight1

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