Redirect this website to design-system

[chosak]

This PR retires this website and redirects all traffic to the CFPB Design System. This is accomplished by a meta redirect tag that also adds UTM parameters (?utm_source=designmanual&utm_medium=redirect&utm_campaign=designsystemlaunch) to the landing URL for tracking in analytics. This PR generates both an index.html and a 404.html so that any pages under /design-manual/ will redirect like this. It also adds a meta robots noindex tag to prevent search engine indexing of these pages.

See https://chosak.github.io/design-manual/ for an example of this in action, and https://github.com/chosak/design-manual for the source code of the proposed change.

A similar PR to this was opened for Capital Framework at https://github.com/cfpb/capital-framework/pull/968.

Checklist

• [x] PR has an informative and human-readable title
• [x] PR has been tagged with the type of change it includes (breaking, enhancement, bug, documentation or internal)
• [x] Changes are limited to a single goal (no scope creep)
• [x] Code can be automatically merged (no conflicts)
• [x] Code follows the standards laid out in the development playbook
• [x] Passes all existing automated tests
• [ ] Any change in functionality is tested
• [ ] New functions are documented (with a description, list of inputs, and expected output)
• [ ] Placeholder code is flagged / future todos are captured in comments
• [ ] Visually tested in supported browsers and devices (see checklist below :point_down:)
• [ ] Project documentation has been updated in this PR with the gulp docs command
• [x] Reviewers requested with the Reviewers tool :arrow_right: