docs: improve AgentKit authentication troubleshooting browsability

[saif-at-scalekit]

Context

PR #745 surfaces agentkit/authentication/troubleshooting in the Go live sidebar. Content is accurate and follows the existing format, but the page is hard to scan when debugging a specific symptom.

Problem

Readers must scroll a long linear page to find their error. Symptom-to-fix mapping is not obvious at a glance.

Proposed improvement

• Add a symptom index at the top (linked anchors or table)
• Group sections by failure phase: connected account status → OAuth redirect → token refresh → provider-specific
• Consider promoting the highest-frequency items to <details> FAQ blocks on agentkit/connections.mdx with links back to deep sections

Acceptance criteria

• Reader can identify their symptom within one screen
• No duplicate content with agentkit/connections.mdx FAQ
• Build + format pass