Automatically move latest changelog to fastlane folder

[wba2hi]

Is your feature request related to a problem? Please describe.
Manually copying over the changelog is error prone and easily forgotten.

Describe the solution you'd like
Add a custom Gradle Task which automates this step. After the creation of the new changelog it will copy the latest changelog entry (only the latest version) and moves it into the fastlane/metadata/android/en-us/changelogs/ folder with the name of "{versionCode}.txt". Formatting should be removed in this step, because markup is not supported.

Describe alternatives you've considered
Continue doing it manually.

Additional context

• Adapt documentation. Add info on which task to execute and also emphasize that the changelog of the fastlane needs to be trimmed, e.g. all "app-unrelated" or "f-droid"-unrelated stuff should be removed.