[Docs] Add this guidance for handling required / optional form fields

[JasonStoltz]

elastic/kibana#195048 (comment)

@MichaelMarcialis Mentioned this guidance, but I don't think we ever added it to our docs. We should take a few minutes to add it. It doesn't need to be perfect, but it's better than not having anything.

This seems like an appropriate place to add it: https://eui.elastic.co/#/forms/form-layouts/guidelines

There's a very brief mention:

Use labels and descriptions
Use a label (and description if needed) for each form field. All form fields should be considered required unless stated alongside the label.

Unfortunately, this guidance is super buried in the docs today. Hopefully in EUI+ we could make it more prominent.,

[MichaelMarcialis]

CCing @joana-cps, as she showed interest in getting it added to our guidelines docs. Also happy to help if time/priorities permit.