New text style rules

[pklaschka]

We use the vale linter in our project from now on. Vale enforces some new style rules. The most important style rule is that you shouldn't write in the first person. Instead, either write in second person ("In this tutorial, you learn how you can write your first own widget" or as a command: "Add a button") or third person ("This creates a button that, when clicked, triggers the onPress event.").

You can find more information about Vale at https://docs.errata.ai/vale/install.

With a few exceptions (like casing in headings and after colons), we follow the Google developer documentation style guide.