Reorganize sessions.md for better flow

Group sections by purpose: mode selection (stateless/stateful/comparison),
transport details (HTTP lifecycle, deployment considerations, stdio),
server configuration (options, ConfigureSessionOptions, DI scopes),
security (user binding), and advanced features (migration, resumability).

Move comparison table near the decision tree. Move deployment footguns
under HTTP transport. Move stateless trade-offs into the stateless
section. Combine 'When to use stateful' and 'When stateful shines'.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: halter73