Add 26.09 release notes

[d0choa]

Adds the release-notes entry for Platform 26.09 to release-notes.md, inserted above 26.06.

What's in 26.09

• Highlights — a major expansion of functional genomics (molecular QTL) data (GTEx v10, plus new IBDverse and MAGE), the GWAS Catalog refresh, two new gene-burden datasources (BRaVa, Genes & Health), updated gnomAD v4.1.1 constraint, richer clinical-trial evidence, cross-ontology disease de-duplication, and a pipeline/deployment/UI overhaul.
• New product features — genetic-quality (QC + heritability) display, redesigned profile-page navigation, and the new metrics page.
• Dataset & schema changes and GraphQL API changes in full.
• Public data source versions and the overall data metrics (measured on the 26.09 release).

Issue references link to opentargets/issues. Numbers are measured on the 26.09 release.

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