Add a summary chapter that shows all of the grammar productions on one page

Author: ehuss

src/SUMMARY.md

@@ -132,6 +132,7 @@
 - [The Rust runtime](runtime.md)
 
 - [Appendices](appendices.md)
+    - [Grammar summary](grammar.md)
     - [Macro Follow-Set Ambiguity Formal Specification](macro-ambiguity.md)
     - [Influences](influences.md)
     - [Test summary](test-summary.md)

src/grammar.md

@@ -0,0 +1,5 @@
+# Grammar summary
+
+The following is a summary of the grammar production rules.
+
+{{ grammar-summary }}