refactor: migrate xenbase and zfin to modular ingests

[kevinschaper]

Summary

• Remove built-in xenbase and zfin transforms from monarch-ingest
• Update ingests.yaml to use pass-through URLs pointing to:
  • xenbase-ingest releases
  • zfin-ingest releases
• Update README with consolidated documentation and modular ingest table
• Update docs/Sources/xenbase.md and docs/Sources/zfin.md to redirect to modular repos

Future Work

All internal koza ingests are expected to be migrated to modular repositories in the same pattern. This PR establishes the approach:

1. Create standalone <source>-ingest repository with justfile-based workflow
2. Configure GitHub Actions to create releases with KGX edge/node TSV artifacts
3. Update ingests.yaml to reference the release artifacts as pass-through URLs
4. Remove the built-in transform code from monarch-ingest
5. Update documentation to point to the modular repository

Remaining ingests to migrate:

• bgee
• ctd
• dictybase
• flybase (if not already in Alliance)
• hgnc
• mgi (if not already in Alliance)
• pombase
• reactome
• rgd (if not already in Alliance)
• sgd (if not already in Alliance)
• string

Related Changes

• Added deprecation notices to legacy repos:
  • zfin-genotype-to-phenotype-ingest
  • zfin-orthology-ingest

Test plan

• Verify xenbase-ingest and zfin-ingest releases exist and are accessible
• Run monarch-ingest download/transform to confirm pass-through URLs work
• Verify no regressions in KG build output

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