dynamic params config have to provide default values

[gildesmarais]

example:

parameters:
  query:
    type: string
    default: "technology"

• migrate existing
• update docs
• make website params form accept/prefill default values,
• should keep alternative values as well
• on website, make use https://developer.mozilla.org/en-US/docs/Web/HTML/Reference/Elements/datalist
• make rspec shared example use defaults