Improve structure and link in the main `README.md` file.

This moves the structure overview to the end to center the reader on the
more substantive sections and removes some redundant content from it.

It also moves the inner headings to be 2nd-level headings instead of
3rd-level (we only had two levels).

Lastly it corrects a link to the goals document.

README.md

@@ -4,19 +4,6 @@ See /LICENSE for license information.
 SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 -->
 
-For more information, see [our goals](docs/project/goals.md).
-
-# Carbon repository overview
-
-The Carbon Language project is an **_experiment_** to explore a possible,
-distant future for the C++ programming language.
-
-Carbon's main repositories are:
-
-*   carbon-lang - Carbon language specification and documentation.
-*   carbon-toolchain - Carbon language toolchain and reference implementation.
-*   carbon-proposals - An archive of reviewed Carbon language proposals.
-
 # Carbon language
 
 The Carbon Language project is an **_experiment_** to explore a possible,
@@ -44,9 +31,9 @@ systematically re-engineer C++ into a near optimal future state along the top
 six priorities, which nonetheless is still reachable through interoperability,
 tooling, automation, and incremental large-scale migration efforts.
 
-For more information, see docs/process/goals.md.
+For more information, see [our goals document](docs/project/goals.md).
 
-### What about other languages?
+## What about other languages?
 
 Other programming languages don't _currently_ address these needs effectively.
 They present interoperability, migration, and performance challenges that make
@@ -77,7 +64,7 @@ stack/ecosystem. However, there is no comparable option for C++ today:
 Carbon explores what it would look like to fill this gap and align it with the
 above priorities.
 
-### Project status
+## Project status
 
 **The project is just getting started.** Everything is at a very early stage. If
 you are hoping to see lots of concrete ideas and plans, you'll probably want to
@@ -107,7 +94,7 @@ languages, but about how to move today's C++ forward effectively. For example, a
 near optimal outcome would be to convince the C++ community to adopt this as its
 official path forward.
 
-### What will make Carbon a compelling future path for C++?
+## What will make Carbon a compelling future path for C++?
 
 We hope that eventually Carbon will provide **significant advantages compared to
 today's C++**. Areas where we think we can most dramatically improve C++ for
@@ -150,3 +137,11 @@ compatibility and interoperability with C++:
 the language as well as large-scale migration for users.** This is extremely
 expensive, and so the bar for Carbon to be a compelling direction for C++ is
 very high.
+
+## Repository structure overview
+
+Carbon's main repositories are:
+
+*   **carbon-lang** - Carbon language specification and documentation.
+*   **carbon-toolchain** - Carbon language toolchain and reference implementation.
+*   **carbon-proposals** - An archive of reviewed Carbon language proposals.