Document opaque types

Closes https://github.com/gleam-lang/language-tour/issues/63

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+<p>
+  <code>let assert</code> is the final way to intentionally crash your Gleam
+  program. It is similar to the <code>panic</code> keyword in that it crashes
+  when the program has reached a point that should never be reached.
+</p>
+<p>
+  <code>let assert</code> is similar to <code>let</code> in that it is a way to
+  assign values to variables, but it is different in that the pattern can be
+  <i>partial</i>. The pattern does not need to match every possible value of the
+  type being assigned.
+</p>
+<p>
+  Like <code>panic</code> this feature should be used sparingly, and likely not
+  at all in libraries.
+</p>