Merge commit '147125babfc18abf586237344d6dab5a4bd1e79f' as 'libs/cli11'

libs/cli11/book/SUMMARY.md

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+# Summary
+
+- [Introduction](/README.md)
+- [Installation](/chapters/installation.md)
+- [Basics](/chapters/basics.md)
+- [Flags](/chapters/flags.md)
+- [Options](/chapters/options.md)
+- [Validators](/chapters/validators.md)
+- [Subcommands and the App](/chapters/subcommands.md)
+- [An advanced example](/chapters/an-advanced-example.md)
+- [Configuration files](/chapters/config.md)
+- [Formatting help output](/chapters/formatting.md)
+- [Toolkits](/chapters/toolkits.md)
+- [Advanced topics](/chapters/advanced-topics.md)
+- [Internals](/chapters/internals.md)