Doc: obsolete need for skeleton files. Fixes `make website' warnings.

Doc: obsolete need for skeleton files.  Fixes `make website' warnings.

Use map source->translationof node names to rewrite any untranslated usages
of already translated nodes.  This occurs eg after a [2] delivery of the
Dutch site, where the untranslated

    news.itexi has @ref{Changes} and
    community.itexi has @ref{Manuals} and @ref{Old downloads}

while these are translated in

    nl/web/manuals.itexi Veranderingen @translationof Changes
    nl/web/manuals.itexi Handleidingen @translationof Manuals
    nl/web/download.itexi Oude downloads @translationof Old downloads

Many thanks for the swift and eager perl help (for how to do `foo =
bar; def bar (): foo ()' where bar happens to be a function) from