Docs: Present how to make a custom function that includes asynchronous lookup

Description

As I wrote in https://github.com/handsontable/hyperformula/issues/9#issuecomment-1029829216:

However, right now, the easiest way at achieve the asynchronicity in the user code outside of HyperFormula. I mean a two-step strategy, in which a function like SHARE_PRICE("AMZN") synchronously resolves to a value from cache (or gives an error if the cache is empty) while asynchronously fetching the current value. When fetching is done, update the cache and reevaluate getCellValue/getSheetValues.

Or, if you need it to run in one pass, simply resolve your async code before feeding the data to Hyperformula.

Links

https://github.com/handsontable/hyperformula/issues/9
https://github.com/handsontable/hyperformula/issues/1119

Type of issue

• Missing documentation
• Error in the documentation
• Request for a demo
• Request for a tutorial, guide etc.
• Other issues, suggestions or ideas