feat(mdbook): allow default configuration (#4207)

## 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

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Signed-off-by: ahmadalguydi <ahmadalgaidy@hotmail.com>