Add feature documentation and artifacts index

artifacts/docs/m4b_authoring_tool.md

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+# M4B Authoring Tool
+
+## Overview
+Audiobookshelf includes a built-in tool to merge multi-file audiobooks into a single M4B file. This tool is accessible via the **Manage** tab of any book.
+
+## "Stream Copy" (Zero Re-encode)
+The most significant improvement to the authoring tool is the **Stream Copy** (Codec: `Copy`) feature.
+
+### Benefits
+- **Quality Preservation**: Since the audio is not re-encoded, there is zero loss in audio quality during the merge.
+- **Speed**: Processing time is reduced from minutes (for re-encoding) to seconds (for disk I/O).
+- **CPU Efficiency**: Minimal processing power is required compared to transcoding.
+
+### How it Works
+1.  **Concatenation**: FFmpeg uses the `concat` demuxer to join the source files into a continuous stream without decoding them.
+2.  **Container Swap**: The stream is wrapped into an MP4/M4B container.
+3.  **Metadata Injection**: Chapters, title, artist, and cover art are injected in a secondary pass, again using `-c copy`.
+
+## Feature Matrix
+
+| Feature | Support |
+| :--- | :--- |
+| **Codecs** | AAC, OPUS, Copy (Stream Copy) |
+| **Chapters** | Automatically generated from Library Item tracks |
+| **Cover Art** | Embedded as a video stream (attached_pic) |
+| **Metadata** | Title, Artist, Album, and Comment tags |
+
+## Usage
+1.  Navigate to a book with multiple audio files.
+2.  Go to **Manage** -> **M4B Encoder**.
+3.  Select **Codec: Copy**.
+4.  Click **Merge**.
+5.  Once complete, a new M4B file will appear in the "Files" tab, and you will be prompted to swap the library item to the new file (optional).
+
+## Requirements
+- Source files must be compatible with the M4B container (typically AAC/M4A/MP3).
+- **Note**: While MP3 files can technically be copied into an M4B container, some players may have better compatibility if MP3s are transcoded to AAC.