Document `incl` folder and including partial docs in other docs

partial docs can go in `incl`

included in another doc with:

```

.. include:: incl/doc-name.rst

```

Start by locating the `incl` folder in the Sphinx documentation source and inspect how existing documents are organized. Document that partial docs belong there and use the shown `.. include:: incl/doc-name.rst` syntax; done means readers can find and understand both conventions.

Written by the indexing model from the issue text.