Add `[Edit this page]` link to doc pages

It would be easier to contribute documentation fixes if pages like http://www.catb.org/esr/reposurgeon/repository-editing.html contained an [Edit this page] link that leads to GitLab, like https://gitlab.com/esr/reposurgeon/-/blob/master/repository-editing.adoc

I found that Asciidoctor support custom headers and footers https://docs.asciidoctor.org/asciidoctor/latest/docinfo/ but haven't found how to get the name of the source .adoc file to generate GitLab URL.