Reorganize the language guide.

- Rename "Expressions" -> "Method Calls".
- Organize "Types" and "Language" into a single linear narrative.
- Mobile-specific navigation to handle the longer guide.
- Rename "Fibers" -> "Concurrency".
- Get rid of duplicate stuff about signatures in "Classes".
- Add next/prev links to each page in the guide.
- Move "Contributing" and "Community" up to the top level.
- Move the precendence table to a separate "Grammar" page.
- Lots of other little stuff.

doc/site/functions.markdown

@@ -1,5 +1,5 @@
 ^title Functions
-^category types
+^category guide
 
 No self-respecting language today can get by without functions—first
 class little bundles of code. Since Wren is object-oriented, most of your code
@@ -161,3 +161,6 @@ to`i`:
     System.print(counter.call()) //> 1
     System.print(counter.call()) //> 2
     System.print(counter.call()) //> 3
+
+<a class="right" href="classes.html">Classes &rarr;</a>
+<a href="variables.html">&larr; Variables</a>