WIP: add remote demos from other packages

[johnnychen94]

This is a parallel work to #159 and focuses only on demos.

This setup would grab all the demos from other packages and build a centralized one, for test purposes, I've added two packages:

• https://juliaimages.org/ImageTransformations.jl/dev/examples/
• https://juliaimages.org/ImageFiltering.jl/dev/demos/

Currently, this is a playground for JuliaDocs/DemoCards.jl#89 (still WIP), but you can check the previews at https://juliaimages.org/previews/PR218/examples

Current unsolved issues:

• the assets/covers are not generated correctly
• section titles might need another name

[johnnychen94]

Yeah it works:

Eventually, I'd like to make it

{
    "theme": "grid",
    "remote": {
        "transformations": "https://github.com/JuliaImages/ImageTransformations.jl.git:docs/examples"
    }
}

or

{
    "theme": "grid",
    "remote": {
        "transformations": {
            "url": "https://github.com/JuliaImages/ImageTransformations.jl.git",
            "subdir": "docs/examples"
        }
    }
}

so that we don't need to introduce the external git submodule commands.