Documentation: fix docbook build errors on developers guide

  * duplicate id in Events_Reference_Output.xml
  * closing tag missing in Integrating.xml
Documentation: replace use of sectN tags by generic section

Publican issues the following message when building the docbook:

*WARNING: Unvalidated tag: 'sect1'. This tag may not be displayed
correctly, may generate invalid xhtml, or may breach Section 508
Accessibility standards.
Documentation: replace 'link' tags by 'xref'

This avoids warnings when publican builds the docbook