Add SDK documentation converted from MDX to javadoc markdown

Import and convert documentation from `modelcontextprotocol` repo:

- `sdk-overview.md`: Features, architecture, and dependencies
- `server.md`: Server implementation and transport providers
- `client.md`: Client implementation, transports, and capabilities

Conversions applied:

- MDX `<Tabs>`/`<Tab>` elements to markdown subsections
- `<Tip>`/`<Note>` callouts to blockquotes
- Internal links updated for javadoc doc-files structure
- Specification links to `modelcontextprotocol.io/specification/latest`

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: jonathanhefner