Enhance README.md with proper markdown formatting and build documentation

[Copilot]

The README.md from commit 96e71a1 lacked markdown structure and build instructions for developers.

Changes

• Markdown structure: Added proper headers, table of contents, code blocks, and list formatting
• Build documentation: Added prerequisites (JDK 17/11, Android SDK 33, NDK versions) and step-by-step build instructions extracted from gradle configuration
• Content organization: Restructured into logical sections (Download, Features, Build Instructions, Statement and License, etc.)
• Link formatting: Converted all project references to proper markdown links
• Minor fixes: Corrected swiftp repository URL typo

Example

Before:

DOWNLOAD APK
If you don't want to compile it yourself...

After:

## Download

### Pre-compiled APK

If you don't want to compile it yourself, you can download a pre-compiled version.

**Note:** Don't click directly—copy the address and paste it into your browser's address bar:

https://od.ixcmstudio.cn/repository/main/ZeroTermux/

The README now serves both end users and developers with clear navigation and actionable build instructions.

Original prompt

Reference: 96e71a1

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