Document how to add routes at build time

[posva]

• callback to allow manipulating generated routes #43 (comment)

Add to https://uvr.esm.is/guide/extending-routes.html#extending-routes-in-config

• More examples
• A guide to EditableTreeNode

[TomT15]

is this also how i would be adding before enter?

[posva]

You can do that with definePage