Edit links

We automatically generate edit links for most sections to make it easier for folks to contribute simple fixes and to help folks find the asciidoc file that generated a particular section. It should appear next to every title-like thing.

Books built with Asciidoctor will automatically pick the correct url for all files and by default doesn’t support overriding edit_url. This is mostly a good thing because the overridden `edit_url`s were out of date in many cases.

Some books override edit_url because the asciidoc files in them are not authoritative. In that case they set edit_url to the "real" place to make the change. Sometimes this is another repository and sometimes it is some code that generates the asciidoc files. These books should add respect_edit_url_overrides to their config. While it isn’t required for AsciiDoc it is required for Asciidoctor.