docs: correct MCP architecture and include it in navigation

[enyst]

The MCP architecture page contains useful material but describes an old action model and overstates result and annotation support. Correct it before including it in Mintlify's navigation and generated LLM index.

• Add MCP under SDK Architecture → SDK Components.
• Correct the definition source link and explain schema validation separately from the MCPToolAction.data wrapper.
• Document text/image conversion and skipped unsupported result blocks.
• Correct annotation fields and distinguish hints from confirmation policy.

Stack order: #815 → this PR → #817. The base branch is an exact upstream copy of #815 at 1d6de7c. GitHub's native stacks do not support cross-fork PRs, so #815 itself cannot join the native stack. Merge #815 first, then rebase/retarget this stack onto main before merging the follow-ups. Kept draft until that dependency is resolved.

Source verification:

• Client bridge.
• Argument validation and wrapper construction, raw schema and read-only hint handling.
• Action wrapper and supported result types.
• Annotation fields, confirmation policies.

GitHub native stack created: this PR is layer 1/2 and #817 is layer 2/2.

Validation: GitHub's Check Broken Links workflow passed. JSON and frontmatter parsed, Python examples parsed with AST, navigation entry and internal destinations checked, git diff --check passed. Local Mintlify validation remains blocked by this environment's uv_interface_addresses error; no local rendered-preview claim.