Static HTML: "Edit or run" for repositories instead of single notebooks

[fonsp]

In some projects, you need to clone the entire repository to run a notebook locally, instead of just downloading and running the notebook file. For example, you might have a repository with a single Project.toml that is used by all notebooks (with Pkg.activate), and some data files (e.g. .csv or .png).

Right now, the static HTML export is only designed for self-contained single-file notebooks:

For simple HTML exports, generated using the GUI button, this is probably the only good option, but for websites generated with PlutoSliderServer, we might want to add an option to customize these instructions, explaining how to clone the entire repository instead.

Thanks @jbrea for the suggestion in JuliaPluto/PlutoSliderServer.jl#52