Clarify TEM / CAM server usage in the documentation

[stefsmeets]

The issue, was my misunderstanding what the use_tem_server and use_cam_server options in settings.yaml meant. I dare say that other people may run into the same issue, and that perhaps an update of the instructions on the website could be useful. Specifically, the website could emphasize the need to run instamatic.temserver prior to running instamatic when using a multi-PC setup. It is easy to mistakenly assume the tem_server and cam_server options talk about TEM External Server from JEOL, and Serval from ASI, respectively.