Describe migration path from PDF Customization folder to plug-in

[infotexture]

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.