ADR: Document decision to provide data migration scripts alongside schema changes

[eecavanna]

Here's a link to the rendered preview of this document: https://github.com/microbiomedata/NMDC_documentation/blob/eecavanna-decisions-0007-mongo-migration-scripts/decisions/0007-mongo-migration-scripts.md

To do:

• [x] Draft ADR
• [x] Populate consulted field
• [ ] Confirm all deciders are OK with the "Decision Outcome" shown

[eecavanna]

The check-links job is failing on GitHub Actions. I created an issue about it, here: https://github.com/microbiomedata/NMDC_documentation/issues/25

Since it is failing even on the main branch (independently of this PR branch), I won't hold up the merging of this PR branch for it.