chore: Updating documentation

[alix-graylog]

Summary

Improve documentation for Graylog secret configuration and add guidance for GitOps users.

Details

• Created new comprehensive guide for configuring Kubernetes secrets required by the Graylog Helm chart, including required keys (MongoDB URI, admin credentials), optional S3 keys, and examples
• Added pre-installation warning in the Helm chart README directing GitOps users to manually manage secrets due to issue GitOps: document existingSecretName, fix root-password restore across upgrades #102
• Updated root README with navigation links to documentation modules
• Updated copyright year to 2026

Linked issues

This addresses issue #102 by documenting the required manual secret configuration for GitOps users.

PR Checklist

• Documentation updated
• Tests added/updated
• This PR includes a new feature
• This PR includes a bugfix
• This PR includes a refactor

Testing Checklist

Static Validation

• Linter check passes: helm lint ./charts/graylog
• Helm renders local template sucessfully: helm template graylog ./charts/graylog --validate

Specific to this PR

• Documentation is complete and properly formatted
• Spelling and grammar verified
• Examples are valid and clear