Moved mods to a dedicated folder.

[imernchavez]

Created FreeCAD-src, step, and stl folders under new mods folder and moved the relevant files into these folders.

Cleaned readme file and reorganized to encourage adding mods under different categories, potentially with a quick description of what each mod does.

In future pull requests I'd like to create categories in the mods readme for :

• "Assembly and access". -Mods that make Thor easier to assemble and service.

• "Functionality". - Mods that add or change functionality of the Thor robot.

  I'd like to share a few designs that would fit nicely into these categories.

[AngelLM]

Great idea, thank you! 😄