The template named generate-basic-index in
{docbook-xsl}/xhtml/autoidx.xsl has a bug that generates a div element
with a wrong xmlns:xlink attribute. See
https://github.com/docbook/xslt10-stylesheets/issues/239.
Rather than fixing docbook-xsl, which would work only in LFS (but we
want to be able to render on other distros), copy the faulty template
to our customization files (lfs-index.xsl), so that this one is used.
We can also simplify it a lot since we don't need all the cases
covered in general docbook-xsl.