Add docs for Width Fieldtype

[joshuablum]

Introduced by https://github.com/statamic/cms/pull/7582

[jeff-hykin]

Megaphone makes the most sense to me as a random outsider.

Broadcast or Loudspeaker would also make sense.

Speaker though would not. Speaker can mean "your personal audio settings", I can say "your speaker" and be referring to a sterosystem, or referring to one of two people in a conversation (the person speaking).