Update README with detailed Unity SQLite native libraries docs and build instructions

[jadware]

Motivation

• Clarify repository purpose by documenting prebuilt SQLite native plugin binaries for Unity and the included SQLite version.
• Provide clear platform support and rebuild instructions so consumers can reproduce or rebuild the native libraries for Android and Windows.

Description

• Replace top-level README heading and add SQLite 3.51.3 version information from sqlite3/jni/src/.
• Add lists of included Android .so, Windows .dll, and WSA .dll targets and note that plugin outputs live under Assets/Plugins/.
• Add Android and Windows build requirements and commands with examples for build.bat and build-windows.bat, and document expected output paths such as Assets/Plugins/Android/arm64-v8a/ and Assets/Plugins/x64/sqlite3.dll.
• Keep existing external references and notes about managed usage and NDK/Visual Studio requirements.

Testing

• No automated tests were run for this documentation-only update.

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Codex Task