Explicit list of the template files, and moved every theme dependencies into a common non-hidden folder

Currently some theme dependencies are hidden into a .theme_imgs and .poppins_font folder, among the gitlab CI specific folders. This PR propose to centralize every images and font into a common tuhh_theme folder, and add the list of theme files in the main README.

Some people are having issues because they don't see that two hidden folders also have to be copied in addition of all the sty and cls files, this may help resolve some issues 😃

PS : maybe the wiki page on how to add a new institute should be updated if this PR is merged ...