Cleaned up the public API and updated the code examples

[mridang]

Which Problems Are Solved

• No single, discoverable entry point for PAT, client‑credentials or private‑key‑JWT auth
• Consumers had to wire up low‑level authenticator classes manually
• Lack of direct links to Zitadel’s integration guides in SDK comments

How the Problems Are Solved

• Added three factory methods in each SDK (withAccessToken, withClientCredentials, withPrivateKey)
• Each method is documented (YARD/Javadoc/sphinx/PHPDoc) with params, return types and links to the corresponding guide:
  • PAT: https://zitadel.com/docs/guides/integrate/service-users/personal-access-token
  • Client Credentials: https://zitadel.com/docs/guides/integrate/service-users/client-credentials
  • Private Key JWT: https://zitadel.com/docs/guides/integrate/service-users/private-key-jwt
• Unified method signatures and formatting across Ruby, Java, Python and PHP

Additional Changes

• Harmonized comment styles and grouping tags per language

Additional Context

None.