Add GUI solo mining guide

I believe I made this guide correctly. This merge request contain the following changes:

1. A new GUI solo mining user guide
2. A link to the guide on the user guides page
3. Several images in the PNG folder that are referenced
4. An update to the prove payment user guide to include information about how to get the transaction key in the GUI

This is my first time using Git desktop, so apologies if I screwed this all up.