webdocs: add "Example" section to all form factor pages

For each form factor page, insert a section "Example" that shows the scattering by uncorrelated, oriented particles for horizontal incidence. Follow the example of Pyramid4.

For each form factor XX, do the following:

• in Examples/ff, replace SasXX.py by XX.py.
  • take code from Pyramid4.py
  • take form factor constructor from SasXX.py
  • make sure all particles have about the same volume
• in Test/Examples/CMakeLists.txt, add a line test_example(ff/XX 2e-10)
• run new test, copy test output to Tests/ReferenceData/ExamplesMini/ff
• add section to Hugo page of XX