nagstamon: Link to the canonical source for installing our tooling
nagstamon: Link to the canonical source for installing our tooling

- Otherwise different places in this manual will diverge from the Get
  Started messaging and that'd be bad.
- Also `brew install` is easier to understand than strange, cryptic
  errors people were getting about Ruby versions when they used `gem
  install`.

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs