exposing extension tutorials to end user

[tballmsft]

It's possible for a github extension to have a number of tutorials, which are listed in the pxt.json as well as referred to in the README.md. See for example:

https://github.com/microsoft/microbit-robot

However, these tutorials are really not visible to the MakeCode user who loads the extension, unless they go to the extra effort to find the README... so....

It would be great if there were an easy way for user to discover any tutorials in an extension. This could be by having a button in the extension toolbox or maybe a dialog (on first load of extension) that would offer to load the tutorials into the gallery view (perhaps right after the user projects?).

[tballmsft]

as the tutorials are *.md files, they might need to be listed in a separate list from "files" in the pxt.json