Add Reference section for MCP tools

[cdebled]

Summary

• Add a new reference/mcp-tools page documenting every tool and resource exposed by the MCP server (Navigation, Reading, Extraction, Interacting, Waiting, State/debugging, Session), sourced from each tool's .description in the browser's src/browser/tools.zig (Tool.definition()), plus save/session_new/session_list/session_close from src/mcp/tools.zig's extra_tools
• Restructure usage/mcp.mdx around the new reference page: keep the practical "how to connect and use it" narrative (stdio/HTTP transport, connecting an AI agent, example prompts, known behaviors), drop the exhaustive per-tool listing now that it lives in the reference
• Update the MCP cross-link in guides/markdown-axtree.mdx to also point at the new tools reference
• ⚠️ Fixes a doc drift on scroll: the tool description in tools.zig#L507 claims the response includes the page URL and title, but execScroll never calls finalizeAction and only returns the requested x/y. Docs now say it returns the scroll position only, matching actual behavior.

This PR is part of a series splitting the new Reference section (CLI, MCP tools, HTTP API, PandaScript) into smaller pieces so each can be reviewed and merged on its own: #76 (CLI reference).

⚠️ To avoid a live broken link, the mcp command cross-links here (reference/mcp-tools.mdx, usage/mcp.mdx) temporarily point at the current /run-locally/commands/mcp page instead of /reference/cli/mcp, which #76 creates. Update those two links once #76 merges.

⚠️ This PR also renames the "## Connecting an AI agent" heading to "## Connect an AI agent", which breaks run-locally/commands.mdx's #connecting-an-ai-agent link in #76 (tracked in its description). Update that anchor when merging this.

[arrufat]

Checked every table against browser main (f55aa1a66) — the content matches the source remarkably well, including the scroll fix. Two things merged into the browser after this PR's snapshot (33763f70) make it stale on arrival; inline comments below.