3l demo state

[1-Bart-1]

fix for issue aenarete/KiteViewers.jl#33

[ufechner7]

Please, check the error when building the documentation:

 Error: 2 docstrings not included in the manual:

    KiteUtils.get_particles_3l :: Union{NTuple{5, Any}, NTuple{6, Any}, NTuple{7, Any}, NTuple{8, Any}}
    KiteUtils.demo_state_4p_3lines :: Union{Tuple{Any}, Tuple{Any, Any}, Tuple{Any, Any, Any}}

These are docstrings in the checked modules (configured with the modules keyword)
that are not included in canonical @docs or @autodocs blocks.

[ufechner7]

minor changes to the documentation required; in particular, update the file docs/src/functions.md