Update documentation for reusable workflows

- Add subsections to Table of Contents for Reusable Workflows section
- Promote Table of Contents to proper heading level (#)
- Add placeholder entry in CHANGELOG for next contribution

Author: numbata

CHANGELOG.md

@@ -2,6 +2,7 @@
 
 ### 0.2.2 (Next)
 
+* Your contribution here.
 * [#15](https://github.com/ruby-grape/danger/pull/15): Extract danger reporting infrastructure into reusable workflows and gem - [@numbata](https://github.com/numbata).
 
 ### 0.2.1 (2024/02/01)

README.md

@@ -4,14 +4,18 @@
 
 [![Build Status](https://travis-ci.org/ruby-grape/danger.svg?branch=master)](https://travis-ci.org/ruby-grape/danger)
 
-## Table of Contents
+# Table of Contents
 
 - [Setup](#setup)
   - [Add Danger](#add-danger)
   - [Add Dangerfile](#add-dangerfile)
   - [Add GitHub Actions Workflows](#add-github-actions-workflows)
   - [Commit via a Pull Request](#commit-via-a-pull-request)
 - [Reusable Workflows](#reusable-workflows)
+  - [Architecture](#architecture)
+  - [Benefits of Reusable Workflows](#benefits-of-reusable-workflows)
+  - [How It Works](#how-it-works)
+  - [Examples](#examples)
 - [License](#license)
 
 ## Setup