Increses spacings between paragraphs and changes link color

[zamith]

Why:

• It was hard to read, especially after code blocks and links were very hard to spot

This change addresses the need by:

• Increasing and standardizing the spacings between code blocks and paragraphs
• Increasing the contrast between the link color and the background

  Link color

For the links I ended up using the color already being used for the link in the banner, here's how it was in terms of contrast:

And how it is with the change:

The red means it does not pass any level of the WCAG 2.0 on contrast ratio, the green means it passes the AA level for any size and the AAA level for sizes larger than 18pt.

Spacings

For the spacings I made all of the most relevant bottom margin use base-spacing instead of ems. I tested in different screen sizes and it seems to be fine. Having more space let's the content breathe a bit more and improves readability. For the code blocks, I believe it was actually a mistake that they had no margin bottom. Here's how it was:

And how it is with the changes:

[sgrif]

/cc @corwinharrell I pulled in some of these today as a stopgap measure, I know you're taking a look at that page tomorrow.

[sgrif]

Looks like changes similar to these were made in https://github.com/sgrif/diesel.rs-website/commit/866eb10e4556dfbb951800171c873fa424bcc116. Thanks, though!