docs: improve github refs documentation

Dependencies

• [2] C3XQSZZL add basic documentation for all plugins
• [3] CIUZRFQK docs: improve github status plugin documentation

Change contents

• replacement in doc/manual/src/plugins/README.md at line 102
  [2.1711]→[2.1711:1731](∅→∅)
  − - `github_endpoint`
  [2.1711]

  [2.1731]
  + - `github_endpoint` (defaults to https://api.github.com)