TemplateMark: The 'this' variable should be documented

[jeromesimeon]

Description

TemplateMark supports a special kind of variable named this which represents the current value being processed in the template. This variable isn't properly documented.

See report and example in: accordproject/template-archive#680

Proposal

Documentation and example for the use of variable {{this}} should go into the TemplateMark guide: https://docs.accordproject.org/docs/markup-templatemark.html

[atharmohammad]

@jeromesimeon I would like to work on the issue.

[jolanglinais]

@jeromesimeon could you label this for what it relates to and its difficulty?