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That page is not here
The documentation moved. It used to be seventy-one hand-written pages describing version 12; it is now the Iris book, twenty-five chapters covering v14, free to read and free to download. Old documentation addresses were not carried across, so if you followed one from a search result or an old bookmark, this is why.
The book
Twenty-four chapters plus a preface and an epilogue. Start at chapter 1 or search the whole thing with the key above.
Code Wiki
Looking for what a specific function does? Google generates this from the source and keeps it in sync commit by commit.
The repository
Source, releases, the changelog, the tutorial, the migration guide, and the issue tracker.