Update images for building the display

[opatut]

In the OpenBikeSensor Discourse, user hcb has provided some high quality photographs for the instructions regarding the assembly of the display unit, hosted here. They agreed to licensing the images as LGPL-3.0 as well.

Please integrate these pictures, where and if reasonable, into the build guide.

[maehw]

I stumbled across this issue on the Discourse.

However, the link to the hosted images yields a "Datei nicht gefunden" (file not found) error message.