docs: add Getting Started page with setup instructions and interactive mode examples

Add a comprehensive Getting Started guide covering Unix/Mac installation via git clone and make, interactive mode usage with code examples, and script execution with a Mandelbrot fractal demo. Also update the site navigation template to link the new page, add placeholder pages for Embedding, Error Handling, and Getting Involved, fix the index page link from "usage.html" to "embedding.html", and reorder the types section in the nav to list Fibers before Functions.

doc/site/index.markdown

@@ -20,7 +20,7 @@ in a Lua-sized package.
  *  **Wren is clean.** The codebase is *small*, but not *dense*. It is readable
     and [lovingly-commented][nan]. It's written in warning-free standard C99.
 
- *  **Wren is fast.** A fast single-pass compiler, tight bytecode, and a
+ *  **Wren is fast.** A fast single-pass compiler to tight bytecode, and a
     compact object representation help Wren [compete with other dynamic
     languages][perf].
 
@@ -30,10 +30,10 @@ in a Lua-sized package.
 
  *  **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][usage].
+    and [an easy-to-use C API][embedding].
 
 [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
+[embedding]: embedding.html