Add guidance on creating source blocks in metanorma-based documents

[ghobona]

The follow guidance should be documented for creating source blocks:

• Place [%unnumbered] above the [source,json] flag so that the document processor does not interpret the source block as a Figure.
• Place [%unnumbered] above the [source,xml] flag so that the document processor does not interpret the source block as a Figure.

An example is below.

[%unnumbered]
[source,xml]
----
<om:phenomenonTime>

<gml:TimeInstant gml:id="ab.ww.402557.wl.1.ti.1">

<gml:timePosition>1981-09-12T00:00:00Z</gml:timePosition>

</gml:TimeInstant>

</om:phenomenonTime>

Time Zone (example is Newfoundland time zone -3:30)

<om:phenomenonTime>

<gml:TimeInstant gml:id="nf.ww.34212.wl.1.ti.1">

<gml:timePosition>1981-09-12T00:00:00-03:30</gml:timePosition>

</gml:TimeInstant>

</om:phenomenonTime>
----

[ghobona]

Guidance documented at https://www.metanorma.org/author/ogc/topics/markup/#unnumbered-blocks