Add scimma to list of alert brokers in index.rst to reflect what's stated in content.html

Update index.rst to say "Public LIGO/Virgo/KAGRA alerts are distributed using NASA’s General Coordinates Network (GCN, https://gcn.nasa.gov) and Scalable Cyberinfrastructure to support Multi-Messenger Astrophysics (SCiMMA, https://scimma.org)" to agree with what's written in content.rst.

I'm not familiar enough with merge requests to make one quickly against the v17 pre-release. I think

index.rst: "Alerts will be distributed through NASA's Gamma-ray Coordinates Network (:term:GCN)."

goes to

index.rst: "Alerts will be distributed using NASA's General Coordinates Network (:term:GCN, https://gcn.nasa.gov) and Scalable Cyberinfrastructure to support Multi-Messenger Astrophysics (:term:SCiMMA, https://scimma.org)."