Fix: md array items heading

[okngneer]

Array items are missing html_id in markdown template.
This results in links generated at render time like autogenerated_heading_2
Meanwhile, array type property links to the correct html_id of the item.

For example, in "examples_md_default/array.md"
root > fruits has [fruits items](#fruits_items)
but a few lines down this is the corresponding items section ## <a name="autogenerated_heading_2"></a>1.1. root > fruits > fruits items