Add headings to overview.md

Author: pierwill

src/overview.md

@@ -17,6 +17,8 @@ So first, let's look at what the compiler does to your code. For now, we will
 avoid mentioning how the compiler implements these steps except as needed;
 we'll talk about that later.
 
+### Invokation
+
 - The compile process begins when a user writes a Rust source program in text
   and invokes the `rustc` compiler on it. The work that the compiler needs to
   perform is defined by command-line options. For example, it is possible to
@@ -26,6 +28,9 @@ we'll talk about that later.
 - Command line argument parsing occurs in the [`rustc_driver`]. This crate
   defines the compile configuration that is requested by the user and passes it
   to the rest of the compilation process as a [`rustc_interface::Config`].
+
+### Lexing and parsing
+
 - The raw Rust source text is analyzed by a low-level lexer located in
   [`rustc_lexer`]. At this stage, the source text is turned into a stream of
   atomic source code units known as _tokens_.  The lexer supports the
@@ -66,6 +71,9 @@ we'll talk about that later.
 - The parser uses the standard `DiagnosticBuilder` API for error handling, but we
   try to recover, parsing a superset of Rust's grammar, while also emitting an error.
 - `rustc_ast::ast::{Crate, Mod, Expr, Pat, ...}` AST nodes are returned from the parser.
+
+### HIR lowering
+
 - We then take the AST and [convert it to High-Level Intermediate
   Representation (HIR)][hir]. This is a compiler-friendly representation of the
   AST.  This involves a lot of desugaring of things like loops and `async fn`.
@@ -77,6 +85,9 @@ we'll talk about that later.
   into the internal representation used by the compiler (`Ty<'tcx>`),
   and using that information to verify the type safety, correctness and
   coherence of the types used in the program).
+
+### MIR lowering
+
 - The HIR is then [lowered to Mid-Level Intermediate Representation (MIR)][mir].
   - Along the way, we construct the THIR, which is an even more desugared HIR.
     THIR is used for pattern and exhaustiveness checking. It is also more
@@ -93,6 +104,9 @@ we'll talk about that later.
   code with the type parameters replaced by concrete types. To do
   this, we need to collect a list of what concrete types to generate code for.
   This is called _monomorphization collection_.
+
+### Code generation
+
 - We then begin what is vaguely called _code generation_ or _codegen_.
   - The [code generation stage (codegen)][codegen] is when higher level
     representations of source are turned into an executable binary. `rustc`