[Cin] Blend Algebra and Program icons

[Andrea paz]

We can put these new variables in the chapter Developer's Section -->
Configuaration Features:

https://cinelerra-gg.org/download/CinelerraGG_Manual/Configuration_Features.html

by adding them in the final part (“Some influential environment
variables:”), accompanied by a description similar to:

"Introduced with Blend Algebra and Blend program plugins; can be used
globally in \CGG{} for new developments."

Suggestions on other sections where to mention them and maybe improve
their description?