Update Documentation to Include Application Extension (AppBuilder) Option

[DaveFullerton]

The documentation for a "New Item" of type "Application"

according to Esri Technical Support is here:

https://enterprise.arcgis.com/en/portal/10.9.1/use/add-items.htm#ESRI_SECTION1_0D1B620254F745AE84F39...

But it does not include the Application Extension (AppBuilder) option.  Also it would be more efficient to have a link to this document in the portal interface (we are using 10.9.1, so maybe this has already happened in a newer release?).  I couldn't find this documentation after extensive searching.  I agree that it is nice to simplify the interface by using "Tips" instead of links to detailed documentation, but in many cases that doesn't make sense.