Build concept exercises + fix concept exercise that does not build.

[heijp06]

I wanted to fix issue #878, but noticed that the doctor-data exercise was not built or tested when the track was built. It turned out that this was the case for all concept exercises. I am assuming this is NOT by design.

I did the following:

• Change the build such that it builds both "concept" and "practice" exercises.
• Fix a concept exercise freelancer-rates that did not build.
• Fix a typo in the text that comes with the freelancer-rates exercise.

freelancer-rates failed to build because of the following line of code:

int rounded_up{std::ceil(after_discount)};

Narrowing conversions (ceil() returns a double) are not allowed with list-initialization.

I tested the build of the track with cmake . && make on Ubuntu 22.04.

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[heijp06]

@vaeng can you review this?

[vaeng]

Thank you for your PR.

My CMake-foo is not the best, I would prefer @ahans or @siebenschlaefer to look at your changes.

Please do not mix different issues into one PR. It is easier to handle pull requests if they only concern one problem at a time.
E.g. one for the docs, one for the build pipeline, and one for the non-compiling exercise (that has a fix in pr #916 with an added comment, because I have not seen your PR earlier).

[heijp06]

@vaeng I reverted my change to the docs and created a separate PR for that: #918 Can you open that?

The change in the non-compiling exercise is not really unrelated to the fix in the pipeline, in that the pipeline can only be fixed if all exercises compile. So I will keep that as is for now. If your PR completes before mine I can fix that by merging.

CI is failing on Windows, I will look into that.