add technical note on influence diagrams

As a subsurface professional, I want to have a note explaining me what is an influence diagram and how to interpret existing ones so that I can build and understand what Prisma exposes.

Influence diagrams are an important concept used in Prisma. They are not the most familiar decision graphical model known by the subsurface community and thus need to be well explained.
In particular, Prisma relying on pyagrum which handles LIMIDs, it is important that the meaning of arcs is clear.

Start by reviewing the existing influence diagrams exposed by Prisma and the pyagrum LIMID terminology mentioned in the issue. The technical note is done when it explains what influence diagrams are, how to interpret existing diagrams, and the meaning of arcs well enough for subsurface professionals to build and understand what Prisma exposes.

Written by the indexing model from the issue text.