[#523] Initial clean of extensions.rst
[#523] Edit writing-extensions.rst part I
[#523] More writing extensions updates
[#523] Toolkit update and other fixes
[#523] Merge extensions and writing extensions
[#523]  Remove erroneous section on plugin ordering
[#523] Improve docs based on Sean's feedback
[#523] More Extension docs reworking
[#523] Remove adding plugin interface code.
[#523] Fix sphinx warnings.
[#523] Minor fixes.
[#523] Reflow and better link to toolkit.
[#523] More cleanups.
[#523] Remove enable plugin section etc.
[#523] Fix formatting in extensions list in docs
[#523]  Some more updates add example etc
[#523] Add example
[#523] Fix formatting
[#523] More improvements
[#523] Remove geospatial and harvester docs form core