Screenshots for demos

[icojb25]

• Added geom.png screenshots of the Demos geometry
• Added README.md's for those Demos which were missing these

Please submit merge requests to the 'master' branch.

Before finalising your merge request, please:

• Ensure that all tests pass with FreeCAD -t TestCfdOF
• Increment the version number in package.xml and metadata.txt

[icojb25]

Start to resolve #93

[oliveroxtoby]

Thanks for this.