Include Meddra mappings from Open Targets

[matentzn]

See here: monarch-initiative/mondo#3122

Action items:

• include open targets mappings in mapping commons as is (with metadata and attribution and all)
• Provide a few lines of documentation on how to update the mapping with a pull request (docs/update-source-mappings.md).

These will then naturally be taking into account by a future "boomer" run (#9). Once the boomer pipeline is implemented, reconciled mappings will go straight back into mondo.owl.