Add GUI solo mining guide

Merged monero-project requested to merge SamsungGalaxyPlayer:master into master

Created by: SamsungGalaxyPlayer

I believe I made this guide correctly. This pull request contains 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

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