Document GEO download and RNA-seq ingestion pipelines in READMEs

[sgiannouk]

• Fix stale Ensembl v109 references to v115 (GEO + main README)
• Align GEO README terminology with main (OE/KO -> Overexpression/Knockout/Knockdown)
• Add brief GEO + RNA-seq pointer section to main README
• Add dedicated pipelines/experiments/README.md for the RNA-seq ingestion pipeline
• Document control_mean_normalized_count in the canonical DE schema

Branch: geo-download-pipeline