[Docs] Add destroy command to CLI documentation

[thedavidprice]

@jtoar I don't believe this happened when the PR was merged. But the yarn rw destroy help info is good and can probably be copy+paste into documentation.

Anything else I might be missing here?

[jtoar]

@thedavidprice I think you got it all--I'll add positionals for consistency since that's what's happening in #594 (I'll add it to there). It'd just be adding something like this to createYargsForComponentDestroy:

builder: (yargs) => {
  yargs.positional('name', {
  desc: `Name of the ${componentName}`,
  type: 'string',
})

[thedavidprice]

duplicate https://github.com/redwoodjs/redwoodjs.com/issues/197