Link to macro syntax page from spec syntax page

[dmnks]

We speak about (built-in) macros and their expansion in various parts of the spec page but never care to actually mention to the reader that there's a dedicated page on macro syntax. Add a bunch of links where appropriate, and since one of those places mentions %dnl, also add %dnl to the macro page (this used to be there, just got missed in the reformatting commit 015c829).

Fixes: #3331

[dmnks]

Now that I look at the placement of the %dnl description again, I wonder if it really belongs to that particular table. Maybe "Spec specific macros" would be a better fit?

[pmatilai]

Uh, how on earth we didn't have %dnl documented in the macro docs? 😳 It certainly belongs in that table, it's not spec-specific although that's where the most typical usage may be.

Putting this into the "Comments" section seems strange. We should have (however brief) section about macros in the syntax, probably as the very first subsection as they're so fundamental to the spec. Maybe something like

### Macros

Each line in the spec is macro-expanded before further processing. The macro syntax and the built-in macros are 
described in <insert link>. Typically there are vast amounts of other macros available for helping with
common packaging tasks.

[dmnks]

Ack, I was too lazy and just added plain links but of course, it felt weird because then how do you decide which mentions of "macros" you turn into links... 😅 So yup, having a small, dedicated intro would be much better. I'll fix it up.