Be smarter about example files

Right now we permit example files to be included as siblings to the docs. (e.g., `connect.md` references `connect_example.cpp`.) These files are considered extraneous by the tool. We should permit all `.cpp` files in the output directories as long as they are start with the same name as a documentation file. Example:

```
m_connect.md // the doc file
m_connect_example.cpp // example file for connect. OK - do not flag.
m_foobar_example.cpp // example for a dead API, perhaps? Bad - should be flagged.
```