feat: Add documentation

docs/index.rst

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+The Stencil template language
+=============================
+
+Stencil is a simple and powerful template language for Swift. It provides a
+syntax similar to Django and Mustache. If you're familiar with these, you will
+feel right at home with Stencil.
+
+.. code-block:: html+django
+
+    There are {{ articles.count }} articles.
+
+    <ul>
+      {% for article in articles %}
+        <li>{{ article.title }} by {{ article.author }}</li>
+      {% endfor %}
+    </ul>
+
+.. code-block:: swift
+
+    let context = Context(dictionary: [
+      "articles": [
+        [ "title": "Migrating from OCUnit to XCTest", "author": "Kyle Fuller" ],
+        [ "title": "Memory Management with ARC", "author": "Kyle Fuller" ],
+      ]
+    ])
+
+    do {
+      let template = try Template(named: "template.stencil")
+      let rendered = try template.render(context)
+      print(rendered)
+    } catch {
+      print("Failed to render template \(error)")
+    }
+
+Contents:
+
+.. toctree::
+   :maxdepth: 2
+
+   templates
+   builtins