Update tokens-and-mail-merge.md

[Clairepickle]

as per import request - section added on SMARTY

Resolve #190

[seancolsen]

Thanks for this @Clairepickle

Sorry it took me a while to merge this. I had to do a bit of head-scratching to figure out where the content actually belongs. (Note that even though #190 has my name on it, that's just because I imported the ticket from Jira where someone else created it.)

And I wanted to roll this work into addressing these tickets as well:

• https://github.com/civicrm/civicrm-dev-docs/issues/316
• https://github.com/civicrm/civicrm-sysadmin-guide/issues/13

Now I'm digging into re-organizing some of this content across all three of our guides. The work you've done here has made this easier. Thanks again! 😁