docs: enhance testing documentation

[heurtematte]

This update improves the setup and testing documentation by adding the required prerequisites, simplifying secret handling for test environments, and documenting the end-to-end validation flow.

Changes include:

• Added a prerequisites section covering all required tools.
• Switched to environment-based secrets to avoid relying on third-party tools during testing.
• Added steps to test the Tailscale token and configure HTTPS.
• Added instructions to install the GitHub App in the organization and verify webhook delivery using the ping event.
• Clarified the echo -n usage when generating base64 encoded values to prevent trailing newline issues.
• Added a webhook verification section after service startup.
• Added a procedure to validate the setup with the Otterdog client.