docs: add MCP observability guidance

[jordansilly77-stack]

Summary

• add an Observability docs page that explains where to instrument MCP requests versus transport requests
• document server middleware as the right layer for prompt/resource/tool request spans
• add suggested low-cardinality span names and attributes for tools, resources, and prompts

Refs #766.

Testing

• uv run mkdocs build --strict

AI assistance was used to draft this documentation, and I reviewed the final change before opening the PR.