Mention themes in PDF customization topics

[infotexture]

Many of the topics that cover PDF customization were written before PDF themes landed in DITA-OT 4.0.

For simple customizations, themes may be a better solution, so related topics should be updated to mention themes, including:

• https://www.dita-ot.org/dev/topics/pdf-customization-plugins
• https://www.dita-ot.org/dev/topics/pdf-customization
• https://www.dita-ot.org/dev/topics/pdf-customization-approaches