Update docs for all accepted styles of metadata

[kkappler]

The documentation could be expanded.
This would be particularly helpful if someone wanted to build a UI based on the mt_metadata schema.

Here are some things that could be updated:

• style could add reference to: name list, number list
• options only refers to controlled vocabulary (which is like typing.Literal), and seems also relevant to some cases of name list.
• ACCEPTED_STYLES and REQUIRED_KEYS in the init.py should be described in detail in the docs
  ACCEPTED_STYLES = [
  "name",
  "url",
  "email",
  "number",
  "date",
  "free form",
  "time",
  "date time",
  "name list",
  "number list",
  "controlled vocabulary",
  "alpha numeric",
  ]
  REQUIRED_KEYS = [
  "attribute",
  "type",
  "required",
  "units",
  "style",
  "description",
  "options",
  "alias",
  "example",
  "default",
  ]