DOC: Update, in particular dag.tree

[hmgaudecker]

We released 0.3.0 without updating the docs extensively.

• Move to myst syntax
• Include dag.tree example notebook, maybe interactively?
• Document exceptions (tackle Move to custom exceptions #32 first)

[MImmesberger]

FYI: I started this already in #17. It seems that there are several formatting issues in the compiled version, though.

[hmgaudecker]

Oops, I did not check the docs when looking at the PR. We should definitely finish this before the next release though!

[hmgaudecker]

For some history of thought, a look at #14 might be helpful. Will close that now as it is outdated.