docs: Work around bug in the docbook xsl stylesheets.

docs: Work around bug in the docbook xsl stylesheets.

docbook-xsl's index generation stylesheet (autoidx.xsl) has a small bug: It
doesn't include xlink in exclude-result-prefixes. Normally just leads to a a
single xmlns:xlink in the <div> containing the index, but because our
customization emits that, xmlns:xlink intead gets added to every element
output by autoidx.xsl below the <div>, totalling around 100kB.

Adding the spurious xmlns:xlink to the <div> ourselves isn't great, but avoids
the duplication.

Reviewed-By: Peter Eisentraut <peter@eisentraut.org>
Discussion: https://postgr.es/m/20220213201618.qz6p6noon3wagr3f%40alap3.anarazel.de