Doc for GSI Vector Index and Composite Vector Index

Add documentation for GSI Optimized Vector Indexes and Composite Vector Indexes.

I’m assuming that the documentation effort will target Server 7.7 first, and be merged into Capella afterwards.

If Server 7.7 is not released as an independent product, this work will be rolled into Server 8.0.

This documentation will mostly take place in the docs-devex repo, which means it can be easily merged from the Server docs to the Capella docs when this feature becomes available in Capella.