docs: badges, live docs link, install instructions

[attitude]

Summary

• Add CI/Docs/PHP-version/License badges under the README tagline for at-a-glance project health and discoverability.
• Replace the vague "live at the docs site" sentence with a direct link to https://attitude.github.io/phpx-server/, keeping the pointer to docs/ sources.
• Add an "Install in your project" section to README.md and a mirrored "Use it in your project" section to docs/docs/getting-started.md, spelling out the non-obvious Composer gotcha: since neither attitude/phpx-server nor attitude/phpx is on Packagist yet, and Composer only reads repositories (and dev-stability requires) from the root composer.json, consumers must declare both VCS repositories and both requires themselves — phpx-server's own repository declaration for phpx doesn't propagate to consumers.

Why

These are adoption blockers: without the install snippet, a first-time consumer hits an unresolvable dependency error with no obvious fix. Badges and a working docs link improve first-impression discoverability.

Test plan

• Verified badge URLs point to existing workflow files (.github/workflows/ci.yml, docs.yml)
• Verified composer.json snippet matches the actual repo/dependency structure (phpx-server requires attitude/phpx dev-main, declares its own VCS repo)
• Rendered both markdown files locally to confirm formatting

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