Building docs for /extensions

I can build docs for the schemas under /schemas just fine with:
`jsonschema2md -o docs/reference -d ./schemas`

How do I build the docs for /extensions without copying over the /schemas sub-directories into /extensions?

I have tried a few different iteration including:
`jsonschema2md -o docs/reference -d ./extensions`
`jsonschema2md -o docs/reference -d ./`
`jsonschema2md -o docs/reference -d extensions/`

Start by reviewing the existing jsonschema2md command that builds docs from ./schemas and compare it with the /extensions directory layout. Investigate how jsonschema2md handles those directories without copying schema subdirectories, using docs/reference as the output target. Done means documenting or enabling a working command for building /extensions documentation.

Written by the indexing model from the issue text.