doc: Build a top-level index of the manuals.

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

@@ -34,6 +34,7 @@
              (gnu packages gawk)
              (gnu packages gettext)
              (gnu packages guile)
+             (gnu packages iso-codes)
              (gnu packages texinfo)
              (gnu packages tex)
              (srfi srfi-19)
@@ -183,7 +184,7 @@ makeinfo OPTIONS."
                        (ice-9 match))
 
           (define (normalize language)
-            ;; Normalize LANGUAGE.  For instance, "zh_CN" become "zh-cn".
+            ;; Normalize LANGUAGE.  For instance, "zh_CN" becomes "zh-cn".
             (string-map (match-lambda
                           (#\_ #\-)
                           (chr chr))
@@ -365,16 +366,19 @@ from SOURCE."
                               (manual "guix")
                               (date 1))
   (define build
+    (with-extensions (list guile-json-3)
       (with-imported-modules '((guix build utils))
         #~(begin
             (use-modules (guix build utils)
+                         (json)
                          (ice-9 match)
                          (ice-9 popen)
                          (sxml simple)
+                         (srfi srfi-1)
                          (srfi srfi-19))
 
             (define (normalize language)          ;XXX: deduplicate
-            ;; Normalize LANGUAGE.  For instance, "zh_CN" become "zh-cn".
+              ;; Normalize LANGUAGE.  For instance, "zh_CN" becomes "zh-cn".
               (string-map (match-lambda
                             (#\_ #\-)
                             (chr chr))
@@ -402,7 +406,10 @@ from SOURCE."
                                    #+(guix-manual-text-domain
                                       source
                                       languages))
-                 (write (gettext ,str "guix-manual"))))
+                   (bindtextdomain "iso_639-3"    ;language names
+                                   #+(file-append iso-codes
+                                                  "/share/locale"))
+                   (write (gettext ,str ,domain))))
 
               (with-language language
                 ;; Since the 'gettext' function caches msgid translations,
@@ -497,6 +504,47 @@ from SOURCE."
                                               ".pdf"))))
                                 "PDF")))))))))
 
+            (define %iso639-languages
+              (vector->list
+               (assoc-ref (call-with-input-file
+                              #+(file-append iso-codes
+                                             "/share/iso-codes/json/iso_639-3.json")
+                            json->scm)
+                          "639-3")))
+
+            (define (language-code->name code)
+              "Return the full name of a language from its ISO-639-3 code."
+              (let ((code (match (string-index code #\_)
+                            (#f    code)
+                            (index (string-take code index)))))
+               (any (lambda (language)
+                      (and (string=? (or (assoc-ref language "alpha_2")
+                                         (assoc-ref language "alpha_3"))
+                                     code)
+                           (assoc-ref language "name")))
+                    %iso639-languages)))
+
+            (define (top-level-index languages)
+              (define title
+                "GNU Guix Reference Manual")
+              (sxml-index
+               "en" title
+               `(main
+                 (article
+                  (@ (class "page centered-block limit-width"))
+                  (h2 ,title)
+                  (div
+                   "The GNU Guix Reference Manual is available in the following
+languages:\n"
+                   (ul
+                    ,@(map (lambda (language)
+                             `(li (a (@ (href ,(normalize language)))
+                                     ,(translate
+                                       (language-code->name language)
+                                       language
+                                       #:domain "iso_639-3"))))
+                           languages)))))))
+
             (define (write-html file sxml)
               (call-with-output-file file
                 (lambda (port)
@@ -508,9 +556,6 @@ from SOURCE."
             (setenv "LC_ALL" "en_US.utf8")
             (setlocale LC_ALL "en_US.utf8")
 
-          (bindtextdomain "guix-manual"
-                          #+(guix-manual-text-domain source languages))
-
             (for-each (lambda (language)
                         (define directory
                           (string-append #$output "/"
@@ -519,7 +564,10 @@ from SOURCE."
                         (mkdir-p directory)
                         (write-html (string-append directory "/index.html")
                                     (language-index language)))
-                    '#$languages))))
+                      '#$languages)
+
+            (write-html (string-append #$output "/index.html")
+                        (top-level-index '#$languages))))))
 
   (computed-file "html-indexes" build))