feat(web): update PostHog connection guide

[siisee11]

One-Line Summary

Refresh the PostHog connection guide so users can find the right project settings, host URL, and personal API key fields without guessing.

User-Facing Changes

• The PostHog guide in the data source connection dialog now walks through Settings -> Project -> General before asking for credentials.
• The guide now explains that the Host URL should be the PostHog app origin like https://us.posthog.com or https://eu.posthog.com, not the SDK api_host collection endpoint.
• The guide now includes screenshot-backed steps for opening Personal API keys and creating a read-scoped key.

Why This Changed

The previous PostHog guide described the required values at a high level, but it did not map them to the current PostHog UI. That left users to infer where Project ID, Host URL, and Personal API key values actually live, and it made it easy to paste the wrong host from the SDK snippet.

How It Changed

• Updated the localized PostHog guide content in apps/web/src/features/data-sources/data-source-connection-guide-dialog.tsx to match the current PostHog settings flow.
• Split the old generic key step into separate steps for opening Personal API keys and creating a read-scoped key.
• Added three new PostHog screenshots under apps/web/public/images/posthog/ so the new steps render with visual references.

Verification

• bun run format
• bun lint --format json apps/web/src/features/data-sources/data-source-connection-guide-dialog.tsx

Video / Screenshot (Optional)

• N/A