Add trailing slashes to various internal doc links.

Fixes #110

docs/content/1.tutorial/default.yml

@@ -237,5 +237,5 @@ body:
       - - -
 
       Here endeth the tutorial! There's lots more to play with, go
-      read [the manual](../manual) if you're interested, and [download
-      jq](../download) if you haven't already.
+      read [the manual](../manual/) if you're interested, and [download
+      jq](../download/) if you haven't already.

docs/content/index/index.yml

@@ -23,5 +23,5 @@ body3: |
   shorter and simpler than you'd expect.
   
 tail: |
-  Go read the [tutorial](tutorial) for more, or the [manual](manual)
+  Go read the [tutorial](tutorial/) for more, or the [manual](manual/)
   for *way* more.