docs(contrib): update links to current URLs - #401

Summary

Several of the links in the CONTRIBUTING.md were either redirecting, pointing to archived content, or pointing to no longer existing content

Details

Specific changes:

• https://wiki.jenkins-ci.org URLs -> https://jenkins.io URLs
  • https://wiki.jenkins-ci.org/display/JENKINS/contributing was effectively 404'ing by redirecting to a Google search; https://www.jenkins.io/participate/ seems like the equivalent new URL
• https://issues.jenkins-ci.org/* -> https://issues.jenkins.io/*
• fix grammar to use "the" when referring to a specific place/noun

Misc Notes

I was reading through the Contributing guidelines before submitting #404 and noticed some broken links and other issues in this docs page, so thought I'd fix and improve them while I'm here

Testing done

• Checked the new links and made sure they work
• See the rendered markdown rich diff of this PR

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
• [n/a] Link to relevant issues in GitHub or Jira
• [n/a] Link to relevant pull requests, esp. upstream and downstream changes
• [n/a] Ensure you have provided tests - that demonstrates feature works or fixes the issue