[heft-config-file] Improve README documentation

[iclanton]

Summary

• Rewrites the minimal README for @rushstack/heft-config-file with comprehensive documentation
• Covers both config file authors (Heft users writing/customizing config files) and API consumers (plugin/loader authors using ProjectConfigurationFile / NonProjectConfigurationFile)
• Documents extends inheritance, all InheritanceType options, the $propertyName.inheritanceType inline annotation syntax, path resolution methods, custom validation, and TestUtilities

Test plan

• Review README renders correctly on GitHub
• Verify all code examples are accurate against the source

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