docs: rewrite OAuth 2.0 guide with PKCE flow, (#21)

examples, and troubleshooting

- Replaced old high-level OAuth 2.0 overview with step-by-step Authorization Code + PKCE flow.
- Added clear separation between Content API (Client Credentials) and User API (Auth Code).
- Included ready-to-run examples in Python (requests) and Node.js (axios).
- Documented PKCE helper utilities (Node, Python).
- Added example authorization URL, token exchange (public vs confidential), and refresh token flows.
- Clarified required headers for User API calls (x-auth-token, x-client-id).
- Added environment base URLs for Pre-Production vs Production.
- Documented important considerations (redirect URI matching, state/nonce usage, scope usage).
- Added common issues & troubleshooting table.

Author: basit3407