docs: refresh README, FAQ, package metadata, and repo social preview

[pietechsolution]

Documentation and metadata pass on the php server SDK. No library code changes.

• README: clearer H1 and opening summary of what the server SDK is responsible for, plus a short navigation strip to the registry page, server SDK docs, the platform site, the browser SDK, and the API reference.
• New "Frequently asked questions" section (4 questions) covering booking, the server/browser split, server-side holds, and bringing your own payment provider. Every statement checked against this repo's source and the existing README.
• Package ecosystem table made consistent with the other SeatLayer SDK repos, all links pointing at the published registry page for each package, with this package marked.
• Package manifest description/keywords extended.
• Added .github/social-preview.png (1280x640). It still has to be uploaded by hand under Settings -> Social preview; committing the file does not set it.

Install instructions were checked against the live registry listing before writing.