Markdown: Don't crash on files with html images, alt text, inside links

If we have code like <a><img></a><a><img></a> and the img has alt text

The old code did
  remove image
  insert alt text
but when we remove the image, the qtextdocument says
  "i don't need the <a> anymore since there's nothing inside"
and then everything breaks because the layout has changed and we're
iterating over "unexisting" text blocks

So instead we just insert the alt text and
since we have selected the image with the cursor it will
replace it correctly

BUGS: 444971