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/qa.markdown

@@ -83,8 +83,10 @@ is that bytecode is a nice trade-off between performance and simplicity. Also:
  *  Many devices like iPhones and game consoles don't allow executing code
     generated at runtime, which rules out just-in-time compilation.
 
- *  I think [fibers](fibers.html) are a really powerful tool, and implementing
-    them is straightforward in a bytecode VM that doesn't use the native stack.
+ *  I think [fibers][] are a really powerful tool, and implementing them is
+    straightforward in a bytecode VM that doesn't use the native stack.
+
+[fibers]: concurrency.html
 
 ## Why is the VM stack-based instead of register-based?