electrostatic cases missing images/results

[prudhomm]

Describe the bug
the images are missing in

• https://docs.feelpp.org/cases/latest/electric/rect/readme.html
• https://docs.feelpp.org/cases/latest/electric/README.html
• https://docs.feelpp.org/cases/latest/electric/quarterturn/readme.html

@Trophime would it be possible to fix this ?

[Trophime]

@prudhomm I actually don't have such image. I shall generate them.

[prudhomm]

But how did you choose the name of the images?

[Trophime]

I just did a copy from thermoelectric test case