Add troubleshooting information for tools (e.g. MsBuild)

We regularly get questions why running a specific tool (e.g. MsBuild) doesn't work with Cake. We should add a troubleshooting section or FAQ entry explaining that Cake only shells out to the external tool and that e.g. getting in to run first on command line can help (also with information how to see what command line Cake calls).

Start in the Cake website documentation and look for the existing troubleshooting or FAQ structure. Add guidance that Cake shells out to external tools such as MsBuild, explain how to verify the tool directly from the command line, and document how to see the command line Cake invokes.

Written by the indexing model from the issue text.