Mark paper-plugin.yml as experimental (#2122 )

While paper plugins and their respective loading strategy have been
integrated into paper for a while now, the developer facing side of
paper plugins is still highly experimental.

Specifically the paper-plugin.yml file structure is still under active
development and its syntax might change without a deprecation cycle.
Its usage for general purpose plugin development is still hence
discouraged.

While developers that are familiar with the caveats of using paper
plugins but decide to do so anyway due to the already existing features
exist, this commit still adds a warning to ensure that developers not
familiar with the system do not accidentally use it for new projects,
preventing frustration down the line with potential missing features or
breaking changes over the usage of a plain plugin.yml file.