Add an overview page for the Solace Upstream policy

Migrate or rewrite the Solace Upstream AI Gateway Policy page (`app/_ai_gateway_policies/solace-upstream/index.md`) to include the overview content from the corresponding plugin page: https://developer.konghq.com/plugins/solace-upstream/

Open app/_ai_gateway_policies/solace-upstream/index.md and compare it with the corresponding Solace Upstream plugin page at developer.konghq.com/plugins/solace-upstream/. Identify the overview content that is missing or needs rewriting, then update the policy page so it includes that overview and matches the source. Done means the policy page presents the relevant overview content clearly.

Written by the indexing model from the issue text.