Document syntax features missing from the user guide

[damithc]

Realized that these features are supported but are missing in our documentation. Some of which are shown in https://markdown-it.github.io/

~subscript~
^superscript^

Starting a list with a given number:

52. item
1. item
1. item

[ang-zeyu]

Here https://markbind.org/userGuide/formattingContents.html#text-styles

We do need to document markdown-it-attrs though, but it has it's own issue already I guess #1106

[damithc]

Here https://markbind.org/userGuide/formattingContents.html#text-styles

Thx. Didn't realize it's already in. That leaves just the list numbering trick to add.