Is there any documentation for the 1.12.2-2.0.0 version of Guidebook anywhere?

Not yet. That's one of the reasons it's still marked beta. The format is pretty much the same, except it now supports nesting links, images and stacks within other tags.

Examples: https://github.com/gigaherz/Guidebook/blob/master/src/main/resources/assets/gbook/xml/guidebook.xml#L48
This is the book I used while testing: https://gist.github.com/gigaherz/0bb26587893ddd47389387499e1fa815

Also this modified version of the Ender-rift book: https://gist.github.com/gigaherz/4848ff14d3dc1008a7e8c8612c8fd4aa

@gigaherz Would you be willing to open the wiki to public editing? I have made some small improvements and would like to continue updating it, but it is restricted to collaborators only.

Done! I didn't realize they were restricted!

is this example updated too: https://github.com/gigaherz/Guidebook/blob/master/examples/config/books/demobook.xml
The game crashes if I use this file as template for my book.

What crash are you getting? Note that the example uses a stage condition which requires the Game Stages mod to be present.

This could be the problem :)
Thanks

The wiki is rather up to date right now, so I guess this issue could be closed.