Add docs for reasonable configuration of lighthouse config

[ChristopherPHolder]

Configurations can be defined in different levels. This issue should give examples and explain them.
Configurations can be enriched or reduced or changed.

LH Configuration

• Define an example for a global LH configuration

  {
    "settings": {
      "onlyCategories": ["performance"]
    } 
  }

  This would only run audits of the performance category in every flow and in each flow in each step
• Define and example for a flow LH based configuration

  {
   "settings": {
     "formFactor": "desktop",
     "throttling": "constants.throttling.desktopDense4G",
     "screenEmulation": "constants.screenEmulationMetrics.desktop",
     "emulatedUserAgent": "constants.userAgents.desktop",
   }
  }

  This will run only one specific audit for a flow.
• Define an example for a step LH based configuration

  {
   "settings": {
      "skipAudits": ["full-page-screenshot"],
   } 
  }

LH Budget

• Define an example for a global LH budget
• Define and example for a flow LH budget
• Define an example for a step LH budget