fix(inbox): point Scouts "Learn more" link to a live docs page

[posthog]

Summary

The Scouts source toggle in Self-driving settings rendered a "Learn more about Scouts" link pointing to https://posthog.com/docs/self-driving, which returns HTTP 404. Users setting up Scouts clicked it and landed on a dead page — 10 distinct users hit it over the last 14 days, with 2 Discord complaints, days before the June 23 Self-driving launch.

This repoints the link to https://posthog.com/blog/self-driving-product, which resolves correctly and describes the Self-driving product that Scouts are part of. That URL is already used by ConfigureAgentsSection.tsx in the same feature area.

Root cause

packages/ui/src/features/inbox/components/SignalSourceToggles.tsx:373 hard-coded docsUrl="https://posthog.com/docs/self-driving" (introduced in #2639). The target docs page was never published.

Verification

• Confirmed https://posthog.com/docs/self-driving returns HTTP 404 via direct fetch.
• Confirmed https://posthog.com/blog/self-driving-product resolves (title: "PostHog Code and the self-driving product").
• grep confirms this was the only remaining code reference to /docs/self-driving.

🤖 Generated with Claude Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit ec6986e.}

[greptile-apps]

_{Reviews (1): Last reviewed commit: "fix(inbox): point Scouts "Learn more" li..." | Re-trigger Greptile}