docs: Reorganize and simplify documentation structure

- Simplify README.md from ~925 to ~217 lines
- Create docs/user-guide.md with detailed feature documentation
- Create docs/architecture.md with technical details and API reference
- Create docs/troubleshooting.md (merged from COMMON_ISSUES.md + README)
- Move DOCKER_INSTALL.md to docs/docker-install.md
- Remove COMMON_ISSUES.md (content merged into troubleshooting.md)
- Add Documentation section with links to all docs

The README now focuses on quick start and installation,
while detailed documentation is organized in docs/ folder.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/docker-install.md

@@ -0,0 +1,91 @@
+# How to Install Docker Engine on Debian / Ubuntu
+
+This guide provides step-by-step instructions for installing Docker Engine on a Debian-based system using Docker's official repository. This is the recommended method as it ensures you get the latest and most stable version.
+
+-----
+
+### Step 1: Set Up the Docker Repository
+
+Next, configure your system to download packages from the official Docker repository instead of the default Debian repository.
+
+#### a. Update the package index and install dependencies:
+
+```bash
+sudo apt-get update
+sudo apt-get install ca-certificates curl
+```
+
+#### b. Add Docker’s official GPG key:
+
+This step ensures that the packages you download are authentic.
+
+```bash
+sudo install -m 0755 -d /etc/apt/keyrings && \
+sudo curl -fsSL https://download.docker.com/linux/debian/gpg -o /etc/apt/keyrings/docker.asc && \
+sudo chmod a+r /etc/apt/keyrings/docker.asc
+```
+
+#### c. Add the repository to your APT sources:
+
+This command automatically detects your Debian version and sets up the repository accordingly.
+
+```bash
+echo \
+  "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/docker.asc] https://download.docker.com/linux/debian \
+  $(. /etc/os-release && echo "$VERSION_CODENAME") stable" | \
+  sudo tee /etc/apt/sources.list.d/docker.list > /dev/null
+```
+
+-----
+
+### Step 2: Install Docker Engine
+
+Now you can install the latest version of Docker Engine and its related components.
+
+#### a. Update the package index again:
+
+```bash
+sudo apt-get update
+```
+
+#### b. Install Docker Engine, CLI, Containerd, and Compose plugin:
+
+The `docker-compose-plugin` package provides the `docker compose` command.
+
+```bash
+sudo apt-get install docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin
+```
+
+-----
+
+### Step 3: Verify the Installation ✅
+
+Run the `hello-world` image to confirm that Docker Engine is installed and running correctly.
+
+```bash
+sudo docker run hello-world
+```
+
+If the installation was successful, you will see a "Hello from Docker\!" message in your terminal.
+
+-----
+
+### Step 4 (Optional): Manage Docker as a Non-root User
+
+To avoid typing `sudo` every time you run a Docker command, add your user to the `docker` group.
+
+#### a. Create the `docker` group (if it doesn't already exist):
+
+```bash
+sudo groupadd docker
+```
+
+#### b. Add your user to the `docker` group:
+
+```bash
+sudo usermod -aG docker $USER
+```
+
+**Important:** You need to **log out and log back in** for this change to take effect. Alternatively, you can run `newgrp docker` in your current terminal session to activate the new group membership.
+
+After this, you can run Docker commands directly (e.g., `docker ps`).