Fix overly constricted page widths for large screens

Summary:
On large screens, the page content is unnecessarily restricted such that
viewing large code blocks causes large numbers of line wraps. We can bump up the
content width for large devices without substantially impacting horizontal eye
movement while reading.

Coincidentally, this simplifies the styling for medium-sized devices as well.

Test Plan:
make serve
Navigate to various pages and verify everything looks good on small, medium, and large
screen sizes.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7193