Improved minio support and updated documentation for running inside the cluster

[obervinov]

Hello everyone! Thank you for this very useful tool, which significantly improves operational safety in my cases.

I recently deployed Popeye inside a Kubernetes cluster as a CronJob and use Minio as a repository for reports.
During testing, I identified a couple of areas for improvement, which I've already validated in my environment. I believe these changes could benefit the wider community:

• Updated in-cluster documentation examples: I've updated the Kubernetes manifests (CronJob) in the documentation to provide clearer and more accurate examples for running Popeye within a Kubernetes cluster.
• Corrected Content-Type for Minio reports: When attempting to view reports in Minio via a browser, I noticed that they were always downloaded locally. This was because the Content-Type of the uploaded objects was consistently set to application/octet-stream. For HTML reports, setting the Content-Type to text/html allows users to open the reports directly in the browser via a direct link. I've implemented a change in s3.go to ensure the correct Content-Type is set for HTML, JSON and YAML reports when uploading to Minio.

Perhaps these enhancements will be valuable to others!