Add more information on mentions

[tillprochaska]

There seems to be a lot of confusion regarding the difference between entities and mentions.

• We should explain mentions in this article: https://docs.alephdata.org/guide/the-basics
• We should explain in which cases are used in XREFs.
  • Mentions are xref’d against entities in other datasets
  • Mentions aren’t xref’d against mentions in other entities
  • Entites aren’t xref’s against mentions in other entities