I've developed a set of tools to pack folder in git repository as mkdocs plugin and assemble site from such plugins.
Using it internally for quite long to have docs in same repo as code while having single site for multiple repos documentation.

Maybe someone will find it useful.

Supported Scenarios:

• Keep documentation close to the code: When a project has multiple repositories, each with its own documentation, the documentation site can be assembled from all repository docs.
• Share a documentation subset across multiple sites: For projects that share some code but maintain independent documentation, the shared documentation part can be distributed as a package and linked to each project site.
• Synchronize the look and feel of multiple sites: When several documentation sites need a unified look and feel, the shared site configuration and UI customizations can be distributed, even though the content differs.
• Bypass the MkDocs requirement to keep all content in the docs_dir: If the docs_dir constraint is limiting, documentation content from outside docs_dir can be linked into the site.

Documentation
Code
PyPI Package