Write working example for "conan install from cmake" section

Googling around I've come across [this stackoverflow question](https://stackoverflow.com/questions/53428536/cmakelists-with-conan). It is true, it is not working because the proposed package `Hello/0.1@memsharded/testing` does not exist in remotes (conan-center).

We should change documentation at https://docs.conan.io/en/latest/howtos/cmake_launch.html to use an existing package (also change README in https://github.com/conan-io/cmake-conan), and maybe add a minimum `main.cpp` file too.

For example, we can use the same example from the _Getting started_ section:
* `Poco/1.9.0@pocoproject/stable`
* `main.cpp`

```cpp
#include "Poco/MD5Engine.h"
#include "Poco/DigestStream.h"

#include

int main(int argc, char** argv)
{
Poco::MD5Engine md5;
Poco::DigestOutputStream ds(md5);
ds << "abcdefghijklmnopqrstuvwxyz";
ds.close();
std::cout << Poco::DigestEngine::digestToHex(md5.digest()) << std::endl;
return 0;
}
```

Open docs at docs.conan.io/en/latest/howtos/cmake_launch.html and the README in conan-io/cmake-conan, then compare their current Hello/0.1@memsharded/testing example with the Getting started example. Update both references to an existing Poco/1.9.0@pocoproject/stable package and include the proposed main.cpp example; done means the documented CMake flow uses an available package and presents a working minimal example.

Written by the indexing model from the issue text.