docs: align API reference with production behavior

[memokailun]

Summary

• Rebuild the README around KITE's information architecture and visual language, with an original Memoket API banner and workflow diagram.
• Correct OAuth/DCR, PATCH, delivery status, retry, error, payload-size, format, and read/write-boundary documentation against the public service and current release implementation.
• Add a prominent compatibility warning for URL-only webhook updates that omit events.
• Add CONTRIBUTING/SECURITY guidance and read-only CI checks for documentation structure and public OAuth metadata.

Verification

• git diff --check
• node scripts/check-docs.mjs
• node scripts/check-public-contract.mjs
• SVG XML validation
• Independent red-team review against the live service and backend release behavior

Maintainer decisions / follow-up

• This repository still has no LICENSE. The README now states that public visibility alone does not grant reuse rights. Please choose an explicit documentation/code-example license before presenting the repository as open source.
• Current release behavior clears events when PATCH omits the field, while some CLI URL-only update flows omit it. Keep the new warning until backend and CLI behavior converge.