It took me way too long to figure out how to link to a headline further down in my blog post within markdown. I hope I can save you some time with this post. Down below I have a headline "Link to headline" and here is the link to it: Link to headline.
The code powering it is the following for the link:
[Link to headline](#linktoheadline)
and the following for the headline:
###Link to headline
The link part should start with # and then the name of the headline without any spaces. That is all there is to it, you can define the text you wish for the link.
Link to headline
I hope this helps you linking within the same document on your ghost blog. If you made it work or had problems with this solution, let me know down below in the comments!