Update docs for "Adding repositories already cloned to disk"

Created by: dadlerj

Needed for:

• https://app.hubspot.com/contacts/2762526/company/554275594

A prospect was confused by our docs at https://docs.sourcegraph.com/admin/repo/add_from_local_disk — seems like they haven't been updated in a long time (they refer to "enabling" a repo).

@uwedeportivo was also confused and unable to get it working.

This page probably just needs a quick refresh.