Align specification with UIP

[iberdinsky-skilld]

https://docs.google.com/document/d/1wCQLXrK1lrV2gYlqmqD2pybTql6_H1dByWIKB5xQFcQ/edit?tab=t.0#heading=h.3949vjfiqczr
not experimental and
https://project.pages.drupalcode.org/ui_patterns/2-authors/1-stories-and-library/ can be used as reference for stories format

@todo:

1. remove experimental words from Readme
2. add links to UIP specs.