add a demo for L0Smooth

I'd like to introduce a demo card for the newly developed L0Smooth algorithm by our OSPP student (@JKay0327) to our demo gallery. There's already comprehensive documentation for it in https://johnnychen94.github.io/ImageSmooth.jl/dev/algorithms/ but adding a small demo here makes it easier to be found.

Adding a demo should be simple enough:

1. add a new julia file to an appropriate folder in docs/examples
2. check the preview with preview_demos("path_to_my_file.jl") (provided by DemoCards)
3. if it looks good enough, run a final build include("docs/make.jl")
4. if nothing suspicious, then open a PR for the julia file.

We have a lot of existing PRs in https://github.com/JuliaImages/juliaimages.github.io/pulls as examples.

@JKay0327 I'll leave this to you.

Add a Julia demo file in an appropriate folder under docs/examples, following existing demo-gallery pull requests as examples. Preview it with preview_demos("path_to_my_file.jl"), then run include("docs/make.jl") for the final build. Done means the L0Smooth demo appears correctly in the gallery without suspicious build output.