Collapse all code blocks in README

[Copilot]

Wraps all uncollapsed code blocks in README.md with <details> tags for better readability and navigation.

Changes

• Training setup: Option 1 (Conda + Pip), Option 2 (Docker), Install Package
• Training commands: Dry run, Stage-1, Stage-2 Video Continue Pretraining
• Evaluation: Chunk-wise Sampling and OV-Encoder Codec evaluation

Example

### Option 1 (Conda + Pip)

<details>
<summary>Click to expand setup commands</summary>

```bash
conda env create -f environment.yml -n ov_encoder
pip install torch==2.7.0 torchvision==0.22.0 ...

```

All code blocks now collapsed by default, matching the style of existing collapsible sections.

Original prompt

还是所有的代码都折叠吧，谢谢

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[Copilot]

Pull request overview

This PR improves the README's readability by collapsing all uncollapsed code blocks in the Training and Evaluation sections using <details> tags, matching the existing collapsible style used in the Quick Start section.

Changes:

• Wrapped all training setup and command code blocks in collapsible <details> tags
• Wrapped all evaluation command code blocks in collapsible <details> tags
• Added descriptive summary text for each collapsible section

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