Sometimes it is more convenient to organise a project in multiple repositories, such as in the case it is made of many self-consistent parts which could be used separately. Therefore, each component needs its documentation and at the same time the overall documentation should remain for the whole project. Since the documentation source files (I am supposing you are using RestructuredText or MarkDown to document your code) are distributed among different repositories, the main project needs to refer to these files without duplications.

If you are using Sphinx or a service like Read the Docs which use it behind the scene, it is possible to easily generate the overall documentation. The first step is to create a new repository for the documentation and add as submodules the repositories containing the documentation you want to include.

git submodule add <repository_url>

In case of Read the Docs with public repositories it is important to note that the repository has to be accessible without authentication. Therefore, when you retrieve the link from GitHub this has to be with the http or https protocol, if you use the git protocol for ssh connection the documentation will not be created correctly.

After you have add all the submodules you have to create an index.rst file in the top level directory of this new package. The file has to contain the toctree element of Sphinx. This will provide the list of document to include during the build process and will generate the TOC. An example if index.rst is:

================
My Documentation
================


.. toctree::
   :maxdepth: 2

   submodule1/docs/index
   submodule2/docs/index

You have to replace the name of the submodule and the path to the index file with the correct one. Following the toctree you could add some text describing the project as a whole or have this in a separate file.

Commit the repository to GitHub and re-build the documentation from Read the Docs (if it is not triggered by the commit) or on command line with Sphinx. You should get the complete documentation.

When a submodule is updated you need to update the submodule reference in your repository because it references a specific commit. To update the submodules do the pull request in each submodule directory, commit and push back to the repository.

Of course it is possible to customise the TOC with many parameters, for more information see the Sphinix documentation on toctree.