Add multi-part pages

[Kaliumhexacyanoferrat]

As a developer of a web application, I would like to mix rendering technologies within one page, so that I can combine complex technology (e.g. plain HTML + CSS) with simple descriptions (such as Markdown).

Example

var scribanPart = ModScriban.Template<MyModel>(Data.FromFile(...));

var modelProvider = (r, h) => new MyModel();

var markdownPart = new MarkdownRenderer(Data.FromFile(...));

var page = MultiPartPage.Create()
                        .Title("My Page")
                        .Description("My Description")
                        .Add("Plain String") // raw string
                        .Add(scribanPart, modelProvider) // via renderer + model
                        .Add((r, h) => markdownPart.Render()); // via lambda

Acceptance criteria

• The functionality is implemented in a new pages module
• The existing PageProvider is moved there as well
• The new builder implements the IContentInfoBuilder interface