insights: document aggregations feature and limitations

Created by: leonore

closes #40805 (closed)

stacked on https://github.com/sourcegraph/sourcegraph/pull/41160

Test plan

sg run docsite for the docs

Run sg start enteprise-codeinsights and make sure learn more link works

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.