Add some screenshots

docs/components/custom-apps/web-app/changelog.md

 # Changelog
 
+![Changelog](../../../assets/changelog.png)
+
 In order to keep our users updated with the latest features developped and bugs that have been fixed, we implemented a changelog feature.
 
 The component retrieves from the dedicated [service](../../services/changelog.md) the changelog of each released version from the most recent to the oldest.

docs/components/custom-apps/web-app/credits.md

 # Credits
 
+![Credits](../../../assets/credits.png)
+
 This page lists the open source tools used by data.grandlyon.com.
 
 The list is fetched from the dedicated [credits](../../services/credits.md) service.

docs/components/custom-apps/web-app/homepage.md

 # Homepage
 
+![Homepage](../../../assets/homepage.png)
+
 This is the landing page of data.grandlyon.com.
 
 The main goal of the page is to encourage the user to discover the catalog of dataset. To do so, it gives two entrypoints: