First round of typographic treatment per #2038

I've implemented the first set of type tweaks to the "About" and "Getting Started" pages.

Updates:
- Headers across both pages now look similar, each larger and slightly heavier.
- Line width now at `720px`.
- Paragraph `font-size` increased to `16px`.
- Paragraph leading (`line-height`) has been increased to `1.6`.
- `article-block` sections now consistent between pages, both containing border separators.