Issue Description:

As pointed out in the Use of letter spacing in DocC forum post, there are various typography related attributes that could be improved to provide a better reading experience for users of Swift-DocC-Render documentation pages.

In order to create the most optimal reading experience, the following CSS attributes should reviewed possibly be updated to work better with the default Helvetica font that is used to display text:

• `line-height`

• `font-weight`

• `letter-spacing`

The `letter-spacing` was the one key one pointed out that might need some adjustments since it may be heavily impacted by the actual font used.