Document how to install third-party plugins in RHDH

Goals:

• Develop content for how to install third-party plugins in RHDH
• Change the title of Installing and viewing dynamic plugins to Installing and viewing plugins in Red Hat Developer Hub
• Change the preface to About plugins chapter
• Add information about third-party plugins in the preface or about chapter
• Restructure the chapters in Installing and viewing dynamic plugins title as follows:
  • Installing and viewing plugins in Red Hat Developer Hub (title)
    • About plugins
    • Installing dynamic plugins in Red Hat Developer Hub
      • Installing dynamic plugins with the Red Hat Developer Hub Operator
      • Installing dynamic plugins using the Helm chart
      • Installing dynamic plugins in an air-gapped environment
    • Installing third-party plugins in Red Hat Developer Hub (new content)
    • Viewing installed plugins