add syntax ch intro

Author: mark-i-m

src/syntax-intro.md

@@ -0,0 +1,8 @@
+# Syntax and the AST
+
+Working directly with source code is very inconvenient and error-prone. Thus,
+before we do anything else, we convert raw source code into an AST. It turns
+out that doing even this involves a lot of work, including lexing, parsing,
+macro expansion, name resolution, conditional compilation, feature-gate
+checking, and validation of the AST. In this chapter, we take a look at all
+of these steps.