docs: Improve Getting Started

[DevTKSS]

GitHub Issue (If applicable): closes #

PR Type

What kind of change does this PR introduce?

• Documentation content changes

What is the current behavior?

• Note about "Uno Check found problems" missing
• MainWindow and Host Property ordering of advises to change them is a bit confusing in this order, as the whole app is building on top of them.
• As the extension docs are expandable in volume, there has been missing a proper overview of the App.xaml.cs contents
• Only Frame Navigation Set-Up was mentioned in the code. As these Getting Started belongs to the uno.extensions, there should at least be that start point from the App.xaml.cs to frame AND Navigation Extension.

What is the new behavior?

• Added Information about Uno Check found problems Banner
• Improved the ordering for Window and Hostbuilder, to build the app step by step and not from end to beginning -> makes it easier to follow the guide
• Added Diff including comments to difference between, what belongs to which feature and at which level in which order we need to put them, when we do not want to create a app to just see where to do this, when we might be migrating or adding from a blank preset app.
• Added short information, how in exchange the Navigation Extension would be used with Builder.

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Contains NO breaking changes
• Associated with an issue (GitHub or internal)

Other information

Internal Issue (If applicable):

[unodevops]

⚠️⚠️ The build 168775 has failed on uno.extensions.