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/getting-started.markdown

@@ -93,9 +93,9 @@ Now run:
     :::sh
     $ ./wren my_script.wren
 
-Neat, right? You're a Wren programmer now! The next step is to [read more
-docs](syntax.html) and learn your way around the language. If you run into
-bugs, or have ideas or questions, any of the following work:
+Neat, right? You're a Wren programmer now! The next step is to [learn the
+language](syntax.html). If you run into bugs, or have ideas or questions, any of
+the following work:
 
  *  **Ask on the [Wren mailing list][list].**
  *  Tell me on twitter at [@munificentbob][twitter].