Allow to add nested paths to layouts

[Kaliumhexacyanoferrat]

As a developer of a web application, I would like to add a nested path to a layout so that I do not need to write all of the boiler plate code myself.

Example

var app = Layout.Create()
                .Add("/api/v1", ...)
                .Add("/api/v2", ...);

instead of

var api = Layout.Create()
                .Add("v1", ...)
                .Add("v2", ...);

var app = Layout.Create()
                .Add("api", api);

Acceptance criteria

• They layout builder supports nested paths
• Trailing, leading and double slashes are ignored
• The feature works for all layout builder extensions defined in the core solution
• If a nested path cannot be added because there is already another handler, an exception will be thrown
• The feature is documented on the website