[test][prefix] Add dummy documentation to assets

This will allow to do more tests in for example

beat/beat.editor>
2 jobs for prefix_documentation in 10 minutes and 31 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Build
passed #166186
docker
build_linux_36

00:10:04

73.0%
passed #166187
macosx
build_macosx_36

00:10:31

73.0%