Add a sub-section on usage of zero-width space

[haqer1]

Zero-width space is currently not mentioned in (e.g.) https://spec.commonmark.org/0.29 at all.

However, it's suggested to be used for certain cases. E.g.:
commonmark/commonmark.js#108 (comment)
commonmark/commonmark.js#148 (comment)

There are also cases where it works for single indicator of inline structure, but not for double:
commonmark/commonmark.js#174

Hence, IMHO, it's worth documenting this.

[haqer1]

FYI, I separated the suggestions into 2 PRs:

1. Add a sub-section for Zero-width space #644: related to first 2 examples in the OP
2. Add a few examples illustrating that zero-width space is not an equivalent of backslash escape #645: related to the last example in the OP (includes item 1. verbatim)

I don't have much more time for this, but these are my quick thoughts on improving documentation in this regard.