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The most popular one is Pandoc: https://pandoc.org/, which provides a command line tool for Windows, Linux, MacOS for documentation conversion.
Please follow the install guide to get the tool ready and the manual to use the tool.
The simplest way to use it after installation is to find the document you want to convert in the command line, and execute: pandoc -t rst SOURCE_FILE.docx -o OUTPUT_FILE.rst
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