Improve e2e integration with stacks such as Rails

[BYK]

I recently tried to integrate Spotlight with a basic Rails app and it was a very cumbersome experience.

• It's not clear that we need the Ruby SDK to get the full value out of Spotlight
• It's not clear that having JS SDK on the frontend provides much more value.
• - We should probably add call to actions both on docs and under certain tabs on the UI
• - We should provide a "Add Sentry JS SDK" snipped there too, or provide a Spotlight bundle that already does this
• It's not clear how/which version of Spotlight to use: Pure HTML, or Electron, or npx @spotlightjs/spotlight + Web Browser. We should pick one golden path and promote it
• Pure HTML integration of Spotlight is very confusing as it requires 3 separate script tags (feat: Add IIFE bundle for single script include #478)
• Spotlight relies too much on running in the same page with the front-end application, such as automatically enabling Sentry JS SDK for itself. Sentry JS SDK should do these automatically when spotlight: true is passed and Spotlight should not rely on being on the same global execution environment with the Sentry JS SDK for best integration (feat: Add spotlightBrowser integration sentry-javascript#13263)

[BYK]

Related: #403

[BYK]

Pure HTML part was handled by #478.

[BYK]

Spotlight relying on running in the same page with Sentry Browser SDK is being addressed in getsentry/sentry-javascript#13263

[BYK]

Related #543.