Documentation feedback and improvements

[BTWS2]

Closes #333
Based on dodona-edu/dodona#3216
Closes #359

[pdawyndt]

@niknetniko: I see the documentation still mentions the feedback hierarchy as tab-contexts-testcases; isn't this something we already changed into more common software testing lingo: unit-cases-test? The old names are probably still supported, but the documentation should preferably mention the recommended attributes.

[bmesuere]

Thank you for your changes! I left a few comments below and inline.

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I'm not convinced that we need an FAQ question about courses vs repositories. In my opinion, adding the question makes things more complicated than they are and may add confusion. A teacher that starts to use Dodona should only know the course concept. More advanced users learn what a repository is. Our new manual does a good job in explaining this.

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I added few comments. Those on the English pages also apply to the Dutch translations.

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In the future, try to make smaller PRs that only add/change a single thing. This makes it a lot easier to review and will speed things up significantly.

[bmesuere]

I agree with Niko here. This fits best in the "soorten oefeningen" section.