Add documentation about single-line-diagram layout & positioning algorithms

[flo-dup]

• Do you want to request a feature or report a bug?
  Feature

• What is the current behavior?
  There is no documentation to explain all the layout/positioning algorithms supported in single-line-diagram. We need a functional description to explain both voltage level layout algorithms and substation layout algorithms.

• What is the motivation / use case for changing the behavior?
  Answer on the website to the questions asked by users of single-line-diagram (by email but also on spectrum)

• Other information
  Note that the existing documentation in single-line-diagram repo should help writing that documentation