docs: add troubleshooting guide for common plugin issues

[Vaishnav88sk]

What this PR does

Adds docs/troubleshooting.md - a new troubleshooting guide covering the most frequently reported issues with the Jenkins OpenTelemetry plugin.

Sections covered

• No traces appearing in the backend (endpoint checklist, collector config)
• OTLP connection refused or timeout (causes table + fix)
• Metrics missing in Prometheus (allow-list config, collector pipeline setup)
• Pipeline logs not visible in Elastic or Loki (log export config, mirror-to-disk)
• ClassCircularityError on plugin install (ClassCircularityError thrown when installing the plugin #1201)
• Traces appear but spans are missing stages (parallel branches, withSpanAttribute)
• Build agents cannot reach the OTLP endpoint (agent-side export explanation)
• Enabling debug logging (log recorder steps + caveat about build logs)

Why

The existing docs (setup-and-configuration.md, build-logs.md) explain how to configure the plugin but do not help users diagnose why something is not working. Issues like #1201, #380, and several others in the tracker show the same root causes recurring without a central reference. This guide consolidates those fixes in one place.

Testing done

Documentation only - no code changes. Verified all cross-links point to existing docs files.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

Fixes #1275