docs: add README HTTP session, timeout, and retry examples

[Mattsface]

Why

Version 0.8.0 adds shared Sessions, timeouts, Session injection, bounded retries, and structured transport exceptions. Users should be able to use those features from the README without reading the full transport document first.

What

• Replace the brief README HTTP note with an HTTP Sessions, Timeouts, and Retries section near Quick Start
• Add copy-and-paste examples for context-manager usage, custom timeouts, Session injection, structured exceptions, and base-exception handling
• Summarize shared Session behavior, ownership rules, default retries, 404 compatibility, lack of response caching, and synchronous API behavior
• Link to docs/http-transport.md for the complete transport details

Tests

• poetry run python public import assertions for Mlb and transport exception classes
• poetry run pytest tests/ --ignore=tests/external_tests (157 passed)
• git diff --check
• Confirmed only README.md changed

Risk and impact

• Minimal
• Documentation-only change; no production Python, HTTP, retry, timeout, Session ownership, exception, endpoint, or version behavior changes
• If examples were wrong, users could copy incorrect usage patterns; examples were checked against the merged 0.8.0 public API and transport docs