Add demo showcasing how to use gridstack.js attributes in CSS

[adiessl]

Description

This PR adds a small demo HTML page showcasing how to use the gs-* attributes gridstack.js provides in CSS, in this case to display the widgets dimensions.

This is a follow-up of this comment: #2854 (comment)

Checklist

• Created tests which fail without the change (if possible)
• All tests passing (yarn test)
• Extended the README / documentation, if necessary

[adumesny]

great, thank you for adding this handy demo! this makes sure I won't ever remove the attr even when not used for css :)