Minor Documentation Improvements

The [documentation ](https://bloomberg.github.io/bde-tools/bbs/howtos/create_library.html#creating-an-application) has a couple of minor improvements to be made in the section about building a simple application.

1. `my_app.m.cpp:20` `bsl::cout` requires `#include `
2. Under "Setup application build dependencies", option 2, "Create a build workspace with the BDE libraries...", it should mention that the top level `CMakeLists.txt` will need to be modified to include `add_subdirectory()`
3. Under "Configure, build and test your application", this command: `bbs_build configure build --target my_app` needs to be run from where the top level `CMakeLists.txt` is. It's a little unclear since the previous command `cd`s into the bde directory

Thanks for putting this toolset and documentation together, it's valuable and helpful

Open the linked create_library.html guide and read the section on building a simple application. Update the example to include bsl_iostream.h, document the required add_subdirectory() change, and clarify where bbs_build configure build --target my_app must be run. Done means all three points are explicit in the documentation.

Written by the indexing model from the issue text.