Allow multiple guides to be published from within one repository
In books/*.yml, allow a guide to define a subdirectory within the repo that should be used when publishing.
For example, in CiviCRM core, we might want to store the Dev Guide, User Guide, and Sysadmin Guide in separate folders within the docs directory. This shouldn't be too hard if we just tell mkdocs to use a subfolder for its build process.
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