refactor(parts): remove part numbers to be consistent

src/part-2-intro.md

@@ -1,4 +1,4 @@
-# Part 2: High-Level Compiler Architecture
+# High-Level Compiler Architecture
 
 The remaining parts of this guide discuss how the compiler works. They go
 through everything from high-level structure of the compiler to how each stage

src/part-3-intro.md

@@ -1,4 +1,4 @@
-# Part 3: Source Code Representation
+# Source Code Representation
 
 This part describes the process of taking raw source code from the user and
 transforming it into various forms that the compiler can work with easily.

src/part-4-intro.md

@@ -1,4 +1,4 @@
-# Part 4: Analysis
+# Analysis
 
 This part discusses the many analyses that the compiler uses to check various
 properties of the code and to inform later stages. Typically, this is what people