Decluttered the root dir (#1895)

## Description

**Please let me know how you feel about this change, I'll make changes
accordingly.**

- #1806

## Motivation and Context

I liked to make the root look slim (I also wanted to create `src` dir
but didnt to avoid creating a huge PR).

## How Has This Been Tested?

_N/A_

## Screenshots (if appropriate):


![image](https://github.com/user-attachments/assets/09c6060a-14a5-4f8d-a346-1a8cf70b40b8)

## Types of changes

_N/A_

---------

Co-authored-by: Niels Laute <niels.laute@live.nl>
Co-authored-by: Marcel W. <marcel.alex.wagner@outlook.com>
Co-authored-by: Marcel Wagner <marcelwagner@microsoft.com>

Author: 3 people

docs/images/WinUIGalleryLanding.PNG renamed to .github/assets/ReadmeHero-dark.png

@@ -91,7 +91,7 @@ extends:
           displayName: Generate Test Signing Certificate
           inputs:
             targetType: filePath
-            filePath: build/GenerateTestPFX.ps1
+            filePath: scripts/GenerateTestPFX.ps1
 
         # Build (Product)
         - task: DotNetCoreCLI@2

docs/images/winui-gallery.png renamed to .github/assets/Screenshot-light.png

@@ -28,7 +28,7 @@ steps:
       displayName: Generate Test Signing Certificate
       inputs:
         targetType: filePath
-        filePath: build/GenerateTestPFX.ps1
+        filePath: scripts/GenerateTestPFX.ps1
 
     - task: DotNetCoreCLI@2
       displayName: Build WinUI Gallery Package

.github/assets/StoreBadge-dark.png

@@ -1,23 +1,22 @@
-![WinUI Gallery hero image](docs/images/WinUIGalleryLanding.PNG)
+![WinUI Gallery hero image](./.github/assets/ReadmeHero-dark.png)
 
-<h1 align="center">
-    WinUI 3 Gallery
-</h1>
+<h1 align="center">WinUI 3 Gallery</h1>
+<p align="center">Companion app for <a style="text-decoration:none" href="https://docs.microsoft.com/windows/apps/winui">WinUI</a> & <a style="text-decoration:none" href="https://github.com/microsoft/WindowsAppSDK">Windows App SDK</a> APIs</p>
 
-[![Build Status](https://dev.azure.com/ms/microsoft-ui-xaml/_apis/build/status%2Fmicrosoft.WinUI-Gallery?branchName=main)](https://dev.azure.com/ms/microsoft-ui-xaml/_build/latest?definitionId=612&branchName=main) [![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg?style=flat-square)](http://makeapullrequest.com) [![GitHub license](https://img.shields.io/badge/license-MIT-blue.svg?style=flat-square)](https://github.com/your/your-project/blob/master/LICENSE)
 
-### Accompanying Sample App for [WinAppSDK](https://github.com/microsoft/WindowsAppSDK)
+This app demonstrates all of the WinUI 3 controls and styles available to make a modern Windows app with the Windows App SDK. It is the interactive companion to the [Fluent Design guidelines](https://docs.microsoft.com/windows/apps/design/basics/) and shows the usage of [WinUI](https://docs.microsoft.com/windows/apps/winui/) through interactive samples, tools and code snippets. 
+
 
-This app demonstrates all of the WinUI 3 controls and styles available to make a modern Windows app with the [Windows App SDK](https://github.com/microsoft/WindowsAppSDK). It is the interactive companion to the [Fluent Design Guidelines](https://docs.microsoft.com/windows/apps/design/basics/) and shows the usage of the [WinUI](https://docs.microsoft.com/windows/apps/winui/) APIs. 
 
 <p align="center">
-<img src="docs/images/winui-gallery.png" alt="WinUI 3 Gallery" width="400"/>
+  <img src="./.github/assets/Screenshot-light.png" alt="WinUI 3 Gallery" width="400"/>
 </p>
 <p align="center">
-<a href="https://apps.microsoft.com/detail/WinUI%203%20Gallery/9P3JFPWWDZRC?launch=true
-	&mode=mini">
-	<img alt="Store badge" src="docs/images/storeBadge.png" width="200"/>
-</a>
+  <a style="text-decoration:none" href="https://apps.microsoft.com/detail/9NGHP3DX8HDX?launch=true&mode=full">
+    <picture>
+      <source media="(prefers-color-scheme: light)" srcset="./.github/assets/StoreBadge-dark.png" width="220" />
+      <img src="./.github/assets/StoreBadge-light.png" width="220" />
+  </picture></a>
 </p>
 
 ## ⭐ Features
@@ -55,7 +54,7 @@ Quick start guide to building the WinUI 3 Gallery:
 
 ### 2. Clone the Repository
 
-```shell
+```powershell
 git clone https://github.com/microsoft/WinUI-Gallery.git
 ```
 

.github/assets/StoreBadge-light.png

@@ -9,4 +9,5 @@ $cert = New-SelfSignedCertificate -Type Custom `
     -TextExtension @("2.5.29.37={text}1.3.6.1.5.5.7.3.3", "2.5.29.19={text}")
 
 $certificateBytes = $cert.Export([System.Security.Cryptography.X509Certificates.X509ContentType]::Pkcs12)
-[System.IO.File]::WriteAllBytes("$PSScriptRoot\WinUI-Gallery-Test.pfx", $certificateBytes)
+mkdir -Force "$PSScriptRoot\..\build" > $null
+[System.IO.File]::WriteAllBytes("$PSScriptRoot\..\build\WinUI-Gallery-Test.pfx", $certificateBytes)