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/grammar.markdown

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+^title Grammar
+^category reference
+
+**TODO: Fill in the rest of the grammar.**
+
+## Precedence
+
+When you mix the different [method call][] syntaxes and other [control flow][]
+operators together, you need to worry about *precedence*—which ones bind
+more tightly than others—and *associativity*—how a series of the
+same kind of call is ordered. Wren mostly follows C, except that it fixes [the
+bitwise operator mistake][mistake]. The full precedence table, from tightest to
+loosest, is:
+
+[method call]: method-calls.html
+[control flow]: control-flow.html
+[mistake]: http://www.lysator.liu.se/c/dmr-on-or.html
+
+<table class="precedence">
+  <tbody>
+    <tr>
+      <th>Prec</th>
+      <th>Operator</th>
+      <th>Description</th>
+      <th>Assoc</th>
+    </tr>
+    <tr>
+      <td>1</td>
+      <td><code>()</code> <code>[]</code> <code>.</code></td>
+      <td>Grouping, Subscript, Method call</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>2</td>
+      <td><code>-</code> <code>!</code> <code>~</code></td>
+      <td>Negate, Not, Complement</td>
+      <td>Right</td>
+    </tr>
+    <tr>
+      <td>3</td>
+      <td><code>*</code> <code>/</code> <code>%</code></td>
+      <td>Multiply, Divide, Modulo</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>4</td>
+      <td><code>+</code> <code>-</code></td>
+      <td>Add, Subtract</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>5</td>
+      <td><code>..</code> <code>...</code></td>
+      <td>Inclusive range, Exclusive range</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>6</td>
+      <td><code>&lt;&lt;</code> <code>&gt;&gt;</code></td>
+      <td>Left shift, Right shift</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>7</td>
+      <td><code>&lt;</code> <code>&lt;=</code> <code>&gt;</code> <code>&gt;=</code></td>
+      <td>Comparison</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>8</td>
+      <td><code>==</code></td>
+      <td>Equals</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>8</td>
+      <td><code>!=</code></td>
+      <td>Not equal</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>9</td>
+      <td><code>&amp;</code></td>
+      <td>Bitwise and</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>10</td>
+      <td><code>^</code></td>
+      <td>Bitwise xor</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>11</td>
+      <td><code>|</code></td>
+      <td>Bitwise or</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>12</td>
+      <td><code>is</code></td>
+      <td>Type test</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>13</td>
+      <td><code>&amp;&amp;</code></td>
+      <td>Logical and</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>14</td>
+      <td><code>||</code></td>
+      <td>Logical or</td>
+      <td>Left</td>
+    </tr>
+    <tr>
+      <td>15</td>
+      <td><code>?:</code></td>
+      <td>Conditional</td>
+      <td>Right</td>
+    </tr>
+    <tr>
+      <td>16</td>
+      <td><code>=</code></td>
+      <td>Assign</td>
+      <td>Right</td>
+    </tr>
+  </tbody>
+</table>