Add documentation page

[paulapreuss]

@JulianBarinton I added a documentation page, there is a link at the top of the navbar. The way that it works is that there is a template under templates/wefe/help_pages for each of the steps, and then they will all be compiled into one help page. So you should write whatever documentation you want for each step into the respective template (e.g. the documentation for WEFEDemand goes into the help_pages/demand.html template). If you want to add general information, you should do that in general.html and not in help_page.html (that template only controls the side bar to jump between steps). We can have a quick chat about this if you have any questions.