These are all the resources that this documentation links to, in alphabetical order.
Bruno Haible and Daiki Ueno. libiconv. August 2020. URL: https://savannah.gnu.org/projects/libiconv/. A software library for working with and converting text. Typically ships on most, if not all, POSIX and Linux systems.
Unicode Consortium. “International Components for Unicode”. April 17th, 2019. URL: https://github.com/hsivonen/encoding_rs The premiere library for not only performing encoding conversions, but performing other Unicode-related algorithms on sequences of text.
Daniel Lemire et. al. “simdutf: Unicode at Gigabytes per Second.” January 28th, 2022. URL: https://github.com/simdutf/simdutf.