docs: add dual-license file (CC BY-SA 4.0 + MIT)

LICENSE

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+# License
+
+This project uses a dual-license model:
+
+## Documentation
+
+All documentation files (Markdown content in the `docs/` directory, the README, and related guides) are licensed under:
+
+**Creative Commons Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)**
+
+You are free to:
+- **Share** — copy and redistribute the material in any medium or format
+- **Adapt** — remix, transform, and build upon the material for any purpose
+
+Under the following terms:
+- **Attribution** — You must give appropriate credit, provide a link to the license, and indicate if changes were made.
+- **ShareAlike** — If you remix, transform, or build upon the material, you must distribute your contributions under the same license as the original.
+
+Full license text: https://creativecommons.org/licenses/by-sa/4.0/legalcode
+
+## Code
+
+All code, scripts, configuration files, and build tools (including `mkdocs.yml`, Python scripts, GitHub Actions workflows, etc.) are licensed under:
+
+**MIT License**
+
+Copyright (c) 2026 elektricM and contributors
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+
+---
+
+## Attribution
+
+This documentation is built from community contributions, particularly from the BC250 Discord community and various open-source resources. See the individual pages and CONTRIBUTING.md for specific attributions.