Proof of concept for single card and multi-card pages

[echappen]

Changes proposed in this pull request:

• Completes layout for single card page—example on /methods/discover/cognitive-walkthrough
• Completes layout for multi-card page—example on /methods/discover
• Also fixes some duplicate package warnings in package.json
• Proof of concept only—remaining content will be done in Port over all remaining content for Methods site #234

Known issues

• We add link icons by evaluating markdown before it’s compiled into html. This doesn’t account for data-based html that doesn’t come from markdown, which is why some headers/links get the icon and others don’t. We should have a system that accounts for the compiled resulting html instead of the markdown, but I consider this outside the scope of this issue (I'll make an issue for it).
• We're using the same USWDS version and theme settings site-wide. Since the old Methods site uses USWDS v2x and this repo uses 3x, there will be slight differences in font size, spacing, etc.

Closes #228
Closes #229

security considerations

None

[jduss4]

Just a couple questions, but they aren't blockers so I went ahead and approved!