You can use @mention{addr} to mention a card, it will tell addr it has a backlinks, e.g. I can mention the guide itself Guide. If the mentioned card has taxon Reference, it will not be a related card, but a references card, let me mention a cool book as an example 10 PRINT CHR$(205.5+RND(1)); : GOTO 10.
You can use @transclude{addr} to include a card, it will tell addr it has a context. This guide itself is an example of transclude.
Embed external web page
Since you can use any HTML, iframe for sure is counted, you can embed external web page, e.g.