feat: new library User Story Mapping

[excalibot]

Story mapping is an effective inception tool to create a product backlog in a visually structured way.

It helps in building a shared understanding, identify gaps in the backlog, see interdependencies, and perform better relative sizing.

Further, it can also help in slicing and release planning activities.

Submitted by @metafeather

Items: Cheatsheet Link, Activity or Goal, Narrative flow step, User task, Release slice

Installation link