Update documentation code examples to include required headers

[ton]

It would be good to update the code examples in the documentation to include the headers that are (minimally) required to use a certain feature. For example, I am reading up on the usage of generators. The only thing preventing me from using them right away in my unit test is that I have to check my local Catch2 include directories and guess which header I would need to include to use the feature.

[LukasP]

Migrating a project to v3 right now and running into the same issue, there is no documentation specifying which feature is in which header, not even a comment in the header files themselves.

[nandanvasudevan]

Let consider this example:

Catch2/docs/benchmarks.md

Lines 47 to 52 in 97313f9

	## Benchmark specification
	Benchmarks can be specified anywhere inside a Catch test case.
	There is a simple and a slightly more advanced version of the `BENCHMARK` macro.
	Let's have a look how a naive Fibonacci implementation could be benchmarked:

Would something like this be more helpful? This ones a little bit on the nose, but just to get the idea.

Required includes

#include <catch2/benchmark/catch_benchmark.hpp>

Benchmark specification

Benchmarks can be specified anywhere inside a Catch test case.
There is a simple and a slightly more advanced version of the BENCHMARK macro.

Let's have a look how a naive Fibonacci implementation could be benchmarked:

[ton]

Personally, I would include the #include directive directly inside the code examples, that is a bit cleaner I would say, but what you propose is also a big improvement.