Maester config - Severity

[merill]

Task for myself to add notes in the docs about maester-config.json

• Update custom tests page to call out how to set severity in tags
• Create a new page on maester-config.json
  • Where the file is located, how Maester looks up the parent path
  • How to create a custom maester-config.json to overide the default severity
  • Diagram showing order of precedence (Pester Tag < default maester-config.json in module < maester-config.json in root (Might need to simplify and remove the one at the root since we embed a default one in the module itself).