Document that the hub imports only the fiberpath docs #6

The hub assembles docs from component repos (mkdocs.yml says "fiberpath, future repos"). Only fiberpath/fiberpath's docs are imported as a permanent subsite. The machine-specific hardware repos in the org (the Marlin winder firmware fork and the winder design notes) describe one particular machine and are expected to move out of the org, so their docs are not wired into deploy.yml. Machine-agnostic compatibility and setup guidance, when written, lives in fiberpath's own docs and arrives through that import. Add this to the README next to the description of what deploy.yml imports.