Redirect methods.18f.gov URLs to guides.18f.gov/methods

[cannandev]

Changes proposed in this pull request:

• Set up a permanent redirect from https://methods.18f.gov to https://guides.18f.gov/methods
• This is step 5 of the Migration guide

Security considerations

This is considered a "soft launch" of the site.

[apburnes]

Closing in favor of #239