rename parts, mv rustc_driver to part 3, make syntax chapter

Author: mark-i-m

Diff for: src/SUMMARY.md

@@ -46,13 +46,17 @@
         - [Salsa](./salsa.md)
     - [Memory Management in Rustc](./memory.md)
 
-- [Part 3: The Compiler Frontend](./part-3-intro.md)
-    - [Lexing and Parsing](./the-parser.md)
-    - [`#[test]` Implementation](./test-implementation.md)
-    - [Panic Implementation](./panic-implementation.md)
-    - [Macro expansion](./macro-expansion.md)
-    - [Name resolution](./name-resolution.md)
-    - [AST Validation](./ast-validation.md)
+- [Part 3: Source Code Representations](./part-3-intro.md)
+    - [The Rustc Driver and Interface](./rustc-driver.md)
+        - [Rustdoc](./rustdoc.md)
+    - [Syntax and the AST](./syntax-intro.md)
+        - [Lexing and Parsing](./the-parser.md)
+        - [`#[test]` Implementation](./test-implementation.md)
+        - [Panic Implementation](./panic-implementation.md)
+        - [Macro expansion](./macro-expansion.md)
+        - [Name resolution](./name-resolution.md)
+        - [AST Validation](./ast-validation.md)
+        - [Feature Gate Checking](./feature-gate-ck.md)
     - [The HIR (High-level IR)](./hir.md)
         - [Lowering AST to HIR](./lowering.md)
         - [Debugging](./hir-debugging.md)
@@ -62,7 +66,7 @@
         - [MIR passes: getting the MIR for a function](./mir/passes.md)
     - [Closure expansion](./closure.md)
 
-- [Part 4: The Type System](./part-4-intro.md)
+- [Part 4: Analysis](./part-4-intro.md)
     - [The `ty` module: representing types](./ty.md)
         - [Generics and substitutions](./generics.md)
         - [`TypeFolder` and `TypeFoldable`](./ty-fold.md)
@@ -105,7 +109,7 @@
         - [Two-phase-borrows](./borrow_check/two_phase_borrows.md)
     - [Parameter Environments](./param_env.md)
 
-- [Part 5: The Compiler Backend](./part-5-intro.md)
+- [Part 5: From MIR to binaries](./part-5-intro.md)
     - [The MIR (Mid-level IR)](./mir/index.md)
         - [MIR optimizations](./mir/optimizations.md)
         - [Debugging](./mir/debugging.md)

Diff for: src/feature-gate-ck.md

@@ -0,0 +1,3 @@
+# Feature Gate Checking
+
+**TODO**: this chapter