Improve and extend documentation for dependency-graph generation

[bigdaz]

• Maybe extract separate chapters for caching and dependency-graph details (to cut down on README bloat)
• Document different patterns for generating a dependency graph:
  • Separate workflow that simply resolves dependencies
  • Generate graph for one or more of your primary workflow steps
• How you can restrict the dependencies reported, and why you might want to.
• Use GITHUB_DEPENDENCY_GRAPH_ENABLED to prevent generation for a build step (#843)
• Dependency scopes and how they aren't currently supported (gradle/github-dependency-graph-gradle-plugin#61)