Feature search doc

[hannesbrandt]

Doxygen documentation page for the search routines

This PR adds search.dox, a documentation page for the search routines. It contains an overview and describes local and partition search in detail as well as the user-defined query points. To reference it as an example, we add documentation of example/spheres as well.

[cburstedde]

Thanks; for the spheres example the reader might get the impression that there would be no 3D version. Can we reformulate in a way that it becomes very clear from the beginning that both 2D and 3D are equally available, and that we just document the 2D version here for convenience?

(This is a general remark that applies to all our examples.)

[hannesbrandt]

That's a good point. I've added references to the 3D counterparts in the 2D examples.

[cburstedde]

The first line shows up as a short title in the list on the web page. The information in parentheses is well meant, but maybe rather put into the text, not the first line, instead?

[hannesbrandt]

I looked at the html output for the default doxygen configuration and it seems that the short title is added only for documentation pages in the "Related Pages" section, while the "Examples" section just gives a list of the example file names without any additional description.