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/core/fiber.markdown

@@ -1,7 +1,9 @@
 ^title Fiber Class
 ^category core
 
-A lightweight coroutine. [Here](../fibers.html) is a gentle introduction.
+A lightweight coroutine. [Here][fibers] is a gentle introduction.
+
+[fibers]: ../concurrency.html
 
 ### Fiber.**new**(function)