update guide on `metadata.manufacturer` & `metadata.component.manufacturer`

[jkowalleck]

update guide with the new features introduced by CycloneDX/specification#379

❗ this issue is only about metadata.manufacturer & metadata.component.manufacturer

• add metatada.manufacturer as "OrganizationalEntity"
  -- fixes #57
• deprecate metatada.manufacture in favour of metadata.component.manufacturer
  -- fixes #346

[stevespringett]

The guide only generalizes supplier and manufacturer. I don't see anything in the guide that needs to be changed.

[jkowalleck]

metatada.manufacturer is new. it needs to be added, not changed.
metatada.manufacture is deprecated now. in case it was mentioned in the guide, id should be chnaged.

did not have time to look into the guide yet, sorry.