Add tree traversal methods to glossary

[wooorm]

Subject of the feature

There are several ways to traverse a tree, typically preorder, postorder, but also inorder and breadth-irst / level-order. See also WikiPedia.

Problem

Projects working with unist typically do this, but either a) don’t document how they do it, or b) document this themselves. Both lead to lacking, incorrect, or incomplete docs.

Expected behaviour

These (most common) types should be documented in the glossary. Maybe with a diagram.

Alternatives

The alternatives (status quo) are shown in problem above.