Code Insights: [FE] Add scaffolding for the insight standalone page

Created by: vovakulikov

Closes https://github.com/sourcegraph/sourcegraph/issues/34758

This PR simply adds some template components for the insight standalone page. I hide this page by local js variable in the main root component just to avoid introducing another experimental feature that we will deprecate/remove anyway in the next two weeks.

Test plan

• Make sure that you can see the new insight page if you enabled the feature flag local variable for it

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.