Add "Edit Online" button

Goal: pages on docs.racket-lang.org should have a button labeled "Edit on GitHub" that opens the scribble file that generated the HTML file that's displaying the button.

This is an issue and not a PR because I'm not sure how to implement it.

Idea 1 (the dream) change Scribble to automatically add "edit on github" links to the drop-down that appears when you click headings (just below "link to this section"). Guess this would only work for cloned packages.

Idea 2 (the less-wild dream) implement a --github <url> flag for the scribble command that uses the given url to generate the "edit on github" links for every page title / section.

Idea 3 (the easy-to-hack) put an @github-link[url] function in a scriblib library. Documentation authors need to explicitly call this function to put a button on their pages. (Instead of a button, maybe add a keyword to @section or @title)

Start by examining Scribble's existing page-title and section handling, then compare the three approaches described: automatic links, a scribble --github flag, or a scriblib @github-link function. Done means documentation pages on docs.racket-lang.org show an "Edit on GitHub" button that opens the Scribble source file generating the displayed HTML.