better align headings and body font size

[kornelski]

I find the main text size (starting from "Rust is blazingly fast and memory-efficient…") a little bit too small for comfortable reading. That's usually not a big deal and I enlarge the text in my browser. However, other text on the page, like headers, and especially the "Rust in production" section (starting from "Hundreds of companies around the world are using Rust…) are relatively much much larger. If I zoom in, they become huge to the point of being hard to read because they're too large.

When I enlarge the page to make the smallest body copy comfortable to read:

other text is blown up in size waaay too much:

Both screenshots are of the same page at the same zoom level, but they're like from completely different worlds.

To me the page would be more readable if text sizes of different sections and headers were matching more closely.

[Zaechus]

The Rust in Production section is especially too large on the mobile site.

[skade]

I added the production-section issue as #772.

[skade]

Closing in favor of #772