"See Also" section inline node body text
Really the best way to do this is with multiple fields. Rather than injecting your related content block in the middle of your body, have 3 separate fields.

1) A teaser field (contains the first paragraph)

2) Related content field (This would likely be an entity reference field. You can render the referenced node with a chosen display mode.)

3) Body field (contains the rest of the post)

The other way to do this would be by dynamically splitting your content (probably with regex) and inserting your other field in the middle. Personally, I wouldn't recommend this second option.