fix: Render child elements in the xref element

[kesara]

This change fixes a bug where child elements were not rendered in the xref element with a section attribute.

This change only affects HTML & PDF outputs.

Fixes #1034

Example outputs

Example XML snippet:

<section anchor="introduction"><name>Introduction</name>

 <t>See Section <xref target="RFC8610" section="2" sectionFormat="bare">The <em>Style</em> of Data Structure Specification</xref> of <xref target="RFC8610">CDDL</xref>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="parens">The <em>Style</em> of Data Structure Specification</xref> of <xref target="RFC8610">CDDL</xref>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="comma">The <em>Style</em> of Data Structure Specification</xref> of <xref target="RFC8610">CDDL</xref>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="of">The <em>Style</em> of Data Structure Specification</xref> of <xref target="RFC8610">CDDL</xref>.</t>
 <t>See <xref target="RFC8610" section="2">The <em>Style</em> of Data Structure Specification</xref> of <xref target="RFC8610">CDDL</xref>.</t>

 <t>See <xref target="introduction">The <em>Style</em> of Data Structure Specification</xref></t>

 <t>See <xref target="RFC8610" section="2"/>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="parens"/>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="comma"/>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="of"/>.</t>
 <t>See <xref target="RFC8610" section="2" sectionFormat="bare"/>.</t>

</section>

Sample HTML outputs

Before proposed changes

After the proposed changes

Sample PDF outputs

Before proposed changes

After the proposed changes