Until now we always displayed the same feature images on the landing
page and it didn't matter if the user was on a desktop or mobile device.
This is now changed so that we display different images on mobile
devices (depending on the screen resolution).
Besides that we also moved the deprecated `themeColor` from the
`Metadata` to the `Viewport` and added it to all pages.
Last but not least we added some missing Android icons, which were
referenced in the `manifest.json` file, but where the icon didn't exist.
This commit adds a "Get Started" guide for the mobile and desktop
version of FeedDeck. The guide describes the basic concepts and settings
of the app to make it easier to get started with the FeedDeck.
This is also used for the review process in the different app stores to
make it easier to get the app approved.
