Augment conceptual docs with Docker-based MCP server deployment guidance

[john-mckillip]

Resolves #147

PR Summary

Augments the conceptual docs with Docker-based MCP server guidance for ASP.NET Core + Streamable HTTP.

What’s Included

1. Added a new conceptual guide for Docker deployment:
   docker.md

2. Added a Deployment section with Docker in conceptual navigation:
   toc.yml

3. Added Docker deployment to the conceptual docs landing page:
   index.md

4. Added a cross-link from transport guidance to Docker deployment:
   transports.md

5. Added Docker deployment as a next step in getting started docs:
   getting-started.md

Docker Guide Coverage

1. Minimal ASP.NET Core MCP server baseline (WithHttpTransport + MapMcp)
2. Multi-stage Dockerfile example
3. Build and run commands
4. Port mapping and endpoint guidance (/mcp and legacy /mcp/sse)
5. Environment variable and secret-handling guidance
6. Health/readiness notes
7. Reverse-proxy/forwarded-header considerations

Scope

1. Docs-only change (conceptual documentation and cross-linking)
2. No new runnable sample project
3. No docker-compose, registry publishing, or cloud deployment playbook additions

Validation

1. dotnet build passed
2. dotnet test passed after installing Node conformance dependencies via npm ci

[jeffhandley]

Thanks @john-mckillip for this contribution and the thorough samples -- a really helpful addition to the deployment docs!

I've merged the latest main to clear the conflicts and pushed a change that reflects the deprecation of the legacy SSE endpoints. With the baseline MapMcp("/mcp"), only the Streamable HTTP endpoint is mapped; legacy SSE (/mcp/sse and /mcp/message) is disabled by default and EnableLegacySse is now obsolete, so the endpoints section now points readers to the Transports guide rather than listing /mcp/sse as available.

I verified the documented server against both ModelContextProtocol.AspNetCore 1.4.1 and 2.0.0-preview.3 -- it builds and serves /mcp on both, and /mcp/sse returns 404 by default on both.

[jeffhandley]

Approving -- verified the documented server builds and serves /mcp against both ModelContextProtocol.AspNetCore 1.4.1 and 2.0.0-preview.3, the DocFX build is clean, and the legacy SSE deprecation is now reflected. Thanks again @john-mckillip!