[2.0] New content for templating: Create custom templates for dotnet …

…new (#2495)

* Add custom templates content

Update

* Add preview warning includes file

* Updates

* Updates

* Updates

* React to feedback

* React to feedback

* Add template links to See also section

* Remove -all|--show-all option

Per dotnet/templating#1033

* React to feedback

* Updates and add explicit creation steps

* Establish tutorial content

* Updates

* Updates

* Updates

* Updates

* Updates

* Updates

* Surface the sample template (dotnet-template-samples repo)

It just merged:
dotnet/dotnet-template-samples#20

* some updates

* readding 1.x, adding tabs

* updated name

* Update dotnet-new.md

* Update create-custom-template.md

includes/core-preview-warning.md

@@ -0,0 +1,2 @@
+> [!WARNING]
+> This topic applies to .NET Core SDK 2.0 Preview 2.