docs(): create code sharing docs

docs/collaboration.md

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+# Collaboration
+
+You can use code server to collaborate with other developers in real time.
+
+## Code sharing using Duckly
+
+[Duckly](https://duckly.com/) allows you to share your code in real-time even with people using different IDEs (like JetBrains and VSCode).
+
+- Cross-IDE support,
+- Real-time typing,
+- P2P encrypted,
+- Voice and audio chat,
+- Terminal sharing
+
+### Installing the Duckly Extension
+
+Duckly uses an extension to provide real time sharing features
+
+1. Install the Duckly extension from OpenVSX on `code-server`.
+```bash
+SERVICE_URL=https://open-vsx.org/vscode/gallery \
+  ITEM_URL=https://open-vsx.org/vscode/item \
+  code-server --install-extension gitduck.code-streaming
+```
+2. Refresh you `code-server` window. You should now be able to see the Duckly extension.
+
+### Sharing with Duckly
+
+As `code-server` is based on VScode, you can follow the steps described on Duckly's [Pair programming with VS Code](https://duckly.com/tools/vscode) page and skip the installation step.