Suggested by Julien Lepiller.
* doc/build.scm (html-manual-indexes)[build]: Add 'with-extensions'.
(translate): Actually honor DOMAIN. Add call to 'bindtextdomain' for
ISO-CODES.
(%iso639-languages): New variable.
(language-code->name, top-level-index): New procedures.
Add call to 'write-html' for OUTPUT/index.html.
* doc/build.scm (html-manual-indexes)[build](sxml-index): Generalize;
add a 'title' and a 'body' parameter and honor them.
(language-index): New procedure.
(write-index): Remove.
(write-html): New procedure.
Use 'write-html' and 'language-index'.
