Add description for the embedded file

[Intelligent2013]

Source issue: metanorma/metanorma#407 (comment)

Note that it says "An optional description can also be added ..." --> MN should require this description as it is needed for accessibility and conformance with PDF/A and PDF/UA!

<pdf:embedded-file filename="image.jpg" src="url(file:///...)" description="My image"/>

[Intelligent2013]

The attachment description can be specified in the adoc as bibliography item title:

[bibliography]
== Bibliography

* [[[file1,attachment:(sources/program.c)]]], Description text for program.c

The Presentation XML:

<bibitem id="file1" hidden="true">
	<formattedref format="application/x-isodoc+xml">Description text for program.c</formattedref>
	<uri type="attachment">sources/program.c</uri>
	<uri type="citation">sources/program.c</uri>
	<docidentifier type="metanorma">[sources/program.c]</docidentifier>
</bibitem>

Resulted PDF:

[Intelligent2013]

Documentation updated in metanorma/metanorma.org#816.