Improve Docs for new Users

[DanielDoehring]

This aims to tackle some of the feedback from #1907.

In particular, I seek to address

2. This overview however does not mention which type of fluxes (i.e. dispatching on normals or integer directions) are needed for which grid

3. Similarly, links [...] for the Structured Mesh to the "Structured mesh with curvilinear mapping" would also be helpful

4. Finally, I haven't seen it explicitly mentioned that TreeMesh makes the domain square?

[github-actions]

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[sloede]

LGTM except for a small typo. It would be great to get some feedback from @knstmrd as well though!

[knstmrd]

Looks good to me! Thanks!