Clarified the help pages a bit

We got feedback that some students were a bit confused by the help pages, so the following changes were made:

Moving the developer section to the bottom of the page, as 99% of people don't need to do anything there. Moving it to the bottom helps students ignore it
Removed the word 'main' from 'main HTML file' as students were confused as there's also a 'main.js' file in some of our boilerplates. (They thought they might need to rename that file to index.html). As there's only one HTML file in an experiment most of the time, the word 'main' was redundant anyway.
Continueing on from the html confusing: students were afraid of overwriting our placeholder HTML, so I added a paragraph telling them it's safe
Lastly, people didn't catch the difference between the location and webdav urls and were trying to use the location url as the webdav share url. Hopefully this small addition helps avoiding that from now on

UiL-OTS-labs / web-experiment-datastore