Option to only build autosummary stubs for rst files explicitly listed in a toctree

Is your feature request related to a problem? Please describe.
We use sphinx to document openpnm. The entire documentation takes ~40 min (much faster if done using -j auto though) to build. For testing purposes, when changing the source files, it becomes almost impractical to see in real time how the changes get rendered.

Describe the solution you'd like
Add an option to be placed in conf.py to only generate autosummary stubs that are explicitly listed in a toctree.

Describe alternatives you've considered
Currently, the workaround we use is to manually rename the rst files (like mymodule.rst to mymodule.rst.foo) for all modules except the one we're changing. This way, sphinx doesn't try to generate stubs for all modules, but rather only for the one we desire.

Start by reading the autosummary and toctree documentation, then locate how autosummary is configured through conf.py and how it selects rst files. Define the option so only stubs for files explicitly listed in a toctree are generated, and verify that unrelated module stubs are skipped while the selected ones still build.