Documentation: Guides section should be merged with the Extending SPL section

[natashadsilva]

The Guides section has step by step instructions on developing Java/C++ operators and creating your first SPL application. Could this section be merged with the Extending SPL section, as there is some overlap? It is good to keep all this info in one section. Also, under "what to do next" we might point to the dev guide and not the KC.