docs: add Server Cards (SEP-2127) usage guide

[tadasant]

What this adds

Documentation for the experimental MCP Server Cards (SEP-2127) + AI Catalog discovery feature you built on experimental-server-card-v2. The feature shipped without docs; this PR adds a guide so the capability is discoverable and usable.

• docs/advanced/server-cards.md — a full guide: what a Server Card / AI Catalog is (leads with an experimental / opt-in warning), a discovery sequence diagram, the card fields, building & serving a card on an ASGI app, static-file hosting, discovery HTTP semantics (DISCOVERY_HEADERS, strong-ETag conditional GET / 304, the /.well-known/ai-catalog.json path with /.well-known/mcp/catalog.json fallback), client-side discovery, and the urn:air: identifier derivation.
• docs_src/server_cards/tutorial00{1..4}.py — the runnable snippets the page embeds via --8<--, written against the real public signatures (build_server_card, mount_server_card, mount_ai_catalog, server_card_entry, discover_server_cards, fetch_ai_catalog/fetch_server_card, the low-level Server).
• mkdocs.yml — one nav line adding the page under Advanced.

Scope

Additive and docs-only. No src/ or tests/ changes — the single non-doc edit is the one-line nav entry. It targets experimental-server-card-v2 so it folds into modelcontextprotocol#2951 alongside the code it documents.

git diff --stat vs the base:

 docs/advanced/server-cards.md        | 170 +++++++++++++++++++++++++++++++++++
 docs_src/server_cards/__init__.py    |   0
 docs_src/server_cards/tutorial001.py |  36 ++++++++
 docs_src/server_cards/tutorial002.py |  29 ++++++
 docs_src/server_cards/tutorial003.py |  11 +++
 docs_src/server_cards/tutorial004.py |  21 +++++
 mkdocs.yml                           |   1 +
 7 files changed, 268 insertions(+)

Verification

• Snippets type-check: uv run pyright docs_src/server_cards → 0 errors, 0 warnings, 0 informations.
• Structural invariants: uv run pytest tests/docs_src/test_shape.py → 451 passed — the new chapter imports cleanly against the current SDK, uses only public mcp modules, and every snippet is included by the page.
• Docs build strictly: scripts/docs/build.sh (Zensical --strict + check_crossrefs.py + llms_txt.py) → exit 0, No issues found; site/advanced/server-cards/index.html renders with all four snippets embedded.

Notes

• Scoped to documenting the feature as built — no behavior changes. In particular I did not touch the 304 branch in discovery_response (ai_catalog.py) that drops the CORS headers; that's already flagged inline on Add experimental Server Cards support (SEP-2127) modelcontextprotocol/python-sdk#2951 and is out of scope here.
• Snippets use the low-level Server for the server-side examples (matching this branch's own tests/experimental/server_card/test_server.py), since build_server_card reads plain identity attributes.

🤖 Opened via Claude Code, on behalf of @tadasant.