Update documentation with new benchmarking approach and decide what to keep of old code

[MarionBWeinzierl]

In #356 , we have simplified the benchmarking/performance checking approached, and updated the README accordingly.

The previous code is still in the repository, as is the documentation page https://github.com/ImperialCollegeLondon/pyrealm/blob/develop/docs/source/development/profiling_and_benchmarking.md .

A decision has to be made whether to keep the old code. The documentation page should be updated and linked into the pyrealm docs again.