nixpkgs/doc: generate consistent docbook ids

to make diff'ing different versions of the generated docs less noisy

doc/doc-support/parameters.xml

···
  <xsl:param name="toc.section.depth" select="0" />
  <xsl:param name="admon.style" select="''" />
  <xsl:param name="callout.graphics.extension" select="'.svg'" />
+ <xsl:param name="generate.consistent.ids" select="1" />
 </xsl:stylesheet>