tutorial: Demonstrate "inline link on first use" xref style

Pursuant to #8 (closed), I thought I would try adding some cross-references from the tutorial to the reference. I wasn't sure of the best style, so I have done a small bit, as an experiment. The principle I was going for in this MR was:

The first time a keyword or macro is referenced, make its identifier a link to the corresponding reference documentation. If necessary, add a small amount of explicit text so that there can be such a link.

If you like this approach, or the one in the other MR I'm about to file, I'm happy to go through the whole tutorial adding these xrefs. (In that case please let me know if you'd like me to have you review the resulting changes, or if I should just merge them.)

Or maybe you have a better plan. I'm happy to take your lead.