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/contributing.markdown

@@ -1,8 +1,7 @@
 ^title Contributing
-^category reference
 
-It should be obvious by now that Wren is under active development and there's
-lots left to do. We'd be delighted to have you participate.
+Wren is under active development and there's lots to do. We'd be delighted to
+have you help!
 
 ## Getting acquainted