docs: complete v0.9 user-facing examples and guide

[zuudevs]

Summary

Completes v0.9 with the remaining user-facing examples/documentation and synchronizes the roadmap before entering the v1.0 release gate.

README

• explains project purpose and frozen v1 scope
• documents C++17 / CMake 3.21+ requirements
• documents supported platforms and methods
• calls out HTTP-only / no HTTPS-TLS in v1
• shows quick-start GET usage
• shows install-tree CMake consumption through cpp_request::cpp_request
• documents reusable Client, response limits, thread-safety, and explicit non-goals
• links API, lifetime, error, response-limit, roadmap, and CMake docs

Getting started guide

Documents:

• one-shot helpers
• persistent Client reuse
• POST bodies
• custom headers and query parameters
• borrowed URL/body lifetime rules
• connect/read/write timeouts
• redirect behavior
• response limits
• structured error handling
• thread-safety expectations
• install-tree consumption

Executable examples

Adds local-only examples for:

• simple GET + error handling
• POST with custom headers, query parameters, and borrowed body
• configured reusable Client with timeouts, redirects, and response limits

All examples target 127.0.0.1:8080; they compile in normal CI but are not network-dependent automated tests.

Roadmap synchronization

• marks v0.8 complete through response-resource hardening in PR feat: bound in-memory response resources #25
• records packaging as PR build: add installable CMake package #26
• records benchmark infrastructure as PR bench: add v0.9 benchmark suite #27
• records modular CMake integration as PR build: modularize CMake configuration #28
• records this PR as the v0.9 examples/documentation completion step
• makes v1.0 integrated release hardening the next milestone

Scope

This PR does not change HTTP behavior or public C++ API semantics. It documents and demonstrates the already-frozen v1 surface.