The website https://devdocs.io includes documentation for libraries, programs, and languages. The list is too long to include here but if you can program it the documentation is likely included on devdocs.io.
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Nice find Tucker, especially for these:
It allows you to install docs for offline use, and was one of my favorite tools when I was in a village with a faulty internet connection.
Nice, this would be very helpful to me. Thanks.