Separate Template Variable handling from Jinja2 information

[wxtim]

Currently much of writing-workflows/jinja2 is generic information about template variables.

Do we want to refactor Jinja2 and Empy pages so that the templating language information is separated from the Cylc template variable handling information?

[oliver-sanders]

Can do.

• templating
  • Jinja2
  • Empy

[wxtim]

assigned 8.x as this is useful but not critical to users moving from 7 to 8.