docs: add licence, contributing, and third-party notices

THIRD_PARTY_NOTICES.md

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+# Third-Party Open Source Notices
+
+This project incorporates the following third-party open source software.
+
+## Direct Dependencies
+
+| Package           | License |
+|-------------------|---------|
+| @rspress/core     | MIT     |
+| react             | MIT     |
+| react-dom         | MIT     |
+| eslint            | MIT     |
+| @eslint/js        | MIT     |
+| typescript-eslint | MIT     |
+| prettier          | MIT     |
+| globals           | MIT     |
+
+RSpress itself incorporates additional third-party code. Please refer to
+the [RSpress repository](https://github.com/web-infra-dev/rspress) for its full dependency list.
+
+## Licence Texts
+
+### MIT
+
+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.