The documentation page just shows a 404 whenever I try to open it.

Thanks for the report! This is actually expected (though a bit unfortunate). From the announcement on Discord:

The old web book link (https://gamma-delta.github.io/HexMod) broke when the mod was moved to the FallingColors org. You can find that book at https://fallingcolors.github.io/HexMod, or preview the new hexdoc book at https://hexcasting.hexxy.media/v/0.10.3.