Add page about authoring VS Code extensions

[JonasHelming]

Can be a very short description and directly link to VS Code API documentation.
Ideally, the Theia extension generator can generate a template.
Should be linked here: https://theia-ide.org/docs/extensions/
Should be in the main menu under "using Theia" => Authoring VS Code extensions

@planger

[planger]

Should this new documentation replace the existing Authoring Plug-ins page?
We can remove it in a subsequent step, but I'm asking because if both are shown, I guess an explanation would be required what's the difference, which can be omitted if we only show the page for VS Code extensions.

[JonasHelming]

on the way, see : #315

[JonasHelming]

#315 should go in asap and then we replace it with this when ready