Missing images in docs

[michakraus]

The docs are missing two images:

┌ Error: invalid local link/image: file does not exist in src/tutorials/linear_wave_equation.md
│   link =
│    @ast MarkdownAST.Image("../images/third_degree_spline.png", "") do
│      MarkdownAST.Text("")
│    end
│
└ @ Documenter ~/.julia/packages/Documenter/nQAq5/src/utilities/utilities.jl:44
┌ Error: invalid local link/image: file does not exist in src/architectures/sympnet.md
│   link =
│    @ast MarkdownAST.Image("../images/sympnet_pendulum_architecture.png", "") do
│      MarkdownAST.Text("")
│    end
│
└ @ Documenter ~/.julia/packages/Documenter/nQAq5/src/utilities/utilities.jl:44

[benedict-96]

These should be taken care of. third_degree_spline and sympnet_architecture are now tikz pictures in the folder of the same name.