docs: revise branching, classes, index, looping, qa, syntax docs and add concurrency/error-handling nav entries

doc/site/index.markdown

@@ -14,19 +14,26 @@ in a Lua-sized package.
     }
 
  *  **Wren is small.** The codebase stays under 5,000 semicolons to keep the
-    language and libraries small enough to fit in your head. You can skim the
-    whole thing in one sitting.
+    language and libraries small enough to fit in your head. You can skim
+    [the whole thing][src] in one sitting.
 
  *  **Wren is clean.** The codebase is *small*, but not *dense*. It is readable
-    and lovingly-commented. It's written in warning-free standard C99.
+    and [lovingly-commented][nan]. It's written in warning-free standard C99.
 
- *  **Wren is fast.** Wren has a fast single-pass compiler, tight bytecode, and
-    a compact object representation.
+ *  **Wren is fast.** A fast single-pass compiler, tight bytecode, and a
+    compact object representation help Wren [compete with other dynamic
+    languages][perf].
 
  *  **Wren is class-based.** There are lots of scripting languages out there,
     but many have unusual or non-existent object models. Wren places
-    classes front and center.
+    [classes][] front and center.
 
  *  **Wren is a scripting language.** Wren is intended for embedding in
     applications. It has no dependencies, a small standard library,
-    and an easy-to-use C API.
+    and [an easy-to-use C API][usage].
+
+[src]: https://github.com/munificent/wren/tree/master/src
+[nan]: https://github.com/munificent/wren/blob/46c1ba92492e9257aba6418403161072d640cb29/src/wren_value.h#L378-L433
+[perf]: performance.html
+[classes]: classes.html
+[usage]: usage.html