This is part of the upstream makefile, simply entering the doc directory and executing the xmlto command, which, in this case, convert the DocBook XML document (yaird.xml) to the HTML file (yaird.html).
This latter is the one which is (correctly) installed by debian/rules.
You were perhaps confused by the fact that this is an "on-the-fly" generation.
You shouldn't look at debian/rules. If you check your build log, you should see something like:
make[2]: Entering directory `/tmp/buildd/ yaird-0. 0.12/doc'
/usr/bin/xmlto xhtml-nochunks yaird.xml
This is part of the upstream makefile, simply entering the doc directory and executing the xmlto command, which, in this case, convert the DocBook XML document (yaird.xml) to the HTML file (yaird.html).
This latter is the one which is (correctly) installed by debian/rules.
You were perhaps confused by the fact that this is an "on-the-fly" generation.