deps: bump hugo/themes/book from 5e15ddb to 7c78a39

Bumps [hugo/themes/book](https://github.com/alex-shpak/hugo-book) from `5e15ddb` to `7c78a39`.
- [Release notes](https://github.com/alex-shpak/hugo-book/releases)
- [Commits](alex-shpak/hugo-book@5e15ddb...7c78a39)

---
updated-dependencies:
- dependency-name: hugo/themes/book
  dependency-type: direct:production
...

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