docs: update documentation for recent merged PRs

[aheritier]

🤖 Automated implementer agent — this comment was posted by the implementer bot from Docker Agentic Platform, not by a human developer

Adds documentation for three user-facing features merged in the last 36 hours.

PR #3720 / #3721 — Markdown image rendering in TUI

• New ### Markdown Images section in docs/features/tui/index.md: documents Kitty graphics protocol rendering, supported URI schemes, security rationale (file:// / sandbox:// rejected), bare-path resolution for agent-generated images, and how to disable via render_images: false.
• Updated render_images description in docs/configuration/user-settings/index.md to cover both tool-result images and Markdown images.

PR #3714 — Per-agent usage details and sidebar info mode

• Rewrote the Agents Panel section to describe the new Compact (default) / Detailed mode toggle and what each shows.
• Added ### Sidebar info mode entry to the /settings Appearance tab description.
• Added sidebar_info_mode: detailed to the YAML config example.
• Added a Cost bullet to the Agent Inspector section list.
• Expanded the /cost command description to mention the By Agent section and its buckets.

PR #3708 — Session browser search by session ID

• Updated the Session Management Browse bullet to note that search matches session IDs in addition to titles (full UUIDs, dash-less variants, and partials).