Codesandbox embedded demos offer a very poor developer experience

[sequba]

We have demos:

• https://hyperformula.handsontable.com/guide/integration-with-react.html#demo
• https://hyperformula.handsontable.com/guide/integration-with-vue.html#demo
• https://hyperformula.handsontable.com/guide/integration-with-angular.html#demo
• https://hyperformula.handsontable.com/guide/integration-with-svelte.html#demo
• https://hyperformula.handsontable.com/guide/custom-functions.html#working-demo

They were not migrated to native HTML+JS, because for these demos it is important to show the directory structure and seperate files in the source code

Ideas:

• migrate to stackblitz (it had poor browser support; need to check if it improved)
• migrate to native demos (like all the other demos in the docs)
• don't show embedded demos at all - just a link to stackblitz

[sequba]

Docs deployed with #1622