Show example explanation

[doup]

Starting in 0.16 the example code documentation will be exposed as markdown content. We need to update the example template to show that content.

Notes:

• This PR shows the content conditionally to be compatible with 0.15, once 0.16 is released we can remove the conditionals.
• Eventually this page could use all the space available and have a better layout for desktop. E.g. maybe the explanation could be on a sidebar.

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Alternatively I can change this PR to put the content above the tabs, like so:

[cart]

I think we should show the example code by default / display the explanation above the code, although I think we need to adjust the styling, as the line height feels too big and the alignment feels off in the screenshot above.

[doup]

I've changed it so the explanation is above the code. There is some horizontal padding so different texts align.

I won't tweak the line-height since the explanation is using the same media-content style as in the docs/news and I think those look good enough. The thing is that in this case these are two paragraphs, and that's why it looks a little bit weird. As we extend/improve the content it will look much better.

[janhohenheim]

If you want a more extreme case to test these changes, see the physics in fixed timesteps example, that one has a wall of text of explanations :)