`graphfcn` sub-modules aren't in API documentation

e.g., search `clip_to_catchment` [here](https://imperialcollegelondon.github.io/SWMManywhere/)

These are the modules within `src/swmmanywhere/graphfcns`

Start with the API documentation linked in the issue and compare it with the modules under src/swmmanywhere/graphfcns, including clip_to_catchment. Find the documentation entry point that controls which sub-modules are included, then update that configuration. Done means the graphfcn sub-modules are visible and searchable in the published API documentation.

Written by the indexing model from the issue text.