feat(adding a new generate option): add single file mode

[petern-sc]

• This enables generating only a single markdown file per schema, rather than including all individual fields as separate files
• Removes the defined in column when in single file mode, since there's no where to link to in this case

fix #531. Addresses part of #296 (Only generating the top level schema)

Please remember to follow the documented commit process, specifcally commit using npm run commit which will construct the commit template required for automated semantic releases.

[petern-sc]

cc @trieloff tagging you from your reply. on #296 if you're free to have a review.
I don't seem to have permissions to assign reviewers.