Write manual section on running timing benchmarks.

[MFraters]

In the world builder I have been using timing benchmarks for a while now, which have been useful. It might be useful to do something similar the future for aspect, but to start out, and as requested in #5804, I wrote a section in the manual on how to set them up.

This pull request adds a section to the manual, explaining how to do these timing benchmarks with cbdr.

[MFraters]

Thanks for the review. Good point about callgrind, that would make a nice addition.

I have addressed your comments.

[MFraters]

@gassmoeller Thanks, I applied your commit

[tjhei]

@gassmoeller You can take a look at and/or reference https://integrated-earth.github.io/2023/02/03/perf.html

[gassmoeller]

@gassmoeller You can take a look at and/or reference https://integrated-earth.github.io/2023/02/03/perf.html

I think this is a comment for @MFraters