Describe migration path from PDF Customization folder to plug-in

Per https://github.com/dita-ot/dita-ot/issues/3826, support for the PDF2 `Customization` directory will be removed from the next major (4.0) release.

To prepare users for that change, the development version of the documentation should be extended with a topic describing options that users have if they are still using `Customization` folders.

Those steps would already be applicable today, and could be included in the docs for any (3.7.x) maintenance release, and would ship with the 4.0 docs at the latest.