feat(mdbook): allow default configuration

[tomatotomata]

Summary

• make the book attribute optional for the mdbook rule
• generate an empty book.toml so mdBook uses its documented default configuration
• add a build test covering a book with only src/SUMMARY.md and chapter sources

This addresses #4194. The issue discussion asked for a PR that demonstrates mdBook can run without a user-provided book.toml; an empty configuration parses to mdBook's defaults, including src as the source directory. I kept the generated file in the package staging tree so the existing input mapping and mdbook_server provider continue to use the same config path. Let me know what you think.

Validation

• git diff --check passed
• inspected mdBook 0.4.44 configuration defaults: an empty TOML file loads src as the source directory
• Bazel execution was not available in this Windows environment, so the new build target is left for the repository CI matrix