Removed bold from links to improve readability

[listochkin]

When Brian did the documentation page overhaul he made most links in it bold. However, on OS X these small bold letters look fuzzy and are harder to read. The problem is more apparent in Firefox that tends to render glyph strokes wider than other browsers. Here I've put up the page fragment showing bold and regular fonts being used for links as it's rendered on my machine:

Also, other pages on Rust website don't use bold for links within text body, and as it is now the documentation page looks out of place.

I know that the whole thing is highly subjective, so if Brian or anyone else feels strongly about the new style I won't mind. Thanks for great work!

[rust-highfive]

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[brson]

Thanks.

The reason they are bold is that I'm thinking of them not just as links but as the titles of entries in a list - the bold is the style of the list.

This is also done on the contributing page, so if changed in one place it should be in both. I rather like the way the contributing page looks though.

I definitely want to remove the bold from the first mention of the book on this page, to give the bold links more weight, like on the contributing page.

The documentation page looks good to me with or without the bold, but with my fondness for the contributing page, I'm on the fence. I'm going to sleep on it.

This might be solved by tweaking the font without removing the boldness. I don't notice fuzziness on Firefox on Windows.

cc @aturon @steveklabnik @AndrewBrinker @bjz you might have opinions on whether to use bold links on the doc and contributing page lists.

Doc page without bold.

Contrib page without bold.

[alilleybrinker]

Fira Sans has a bit of a beefy bold. There should be a nice middle ground of using font-weight: 600 instead of the usual font-weight: 700. That way it's bolded, but not as harshly as it is now.

[alilleybrinker]

Huh. It doesn't look like the font is served at a semibold weight. I'll see if something else looks better.

[brendanzab]

I agree with the removal of the use of a normal weight for small font sizes - this looks much easier to read, because the counters are more open. There is also enough hierarchy present in the different colour as opposed to the body text - making the links bold as well makes it seem a tad busy.

[brson]

Thanks @listochkin @AndrewBrinker @bjz.

I'm merging this on its own merits and will think about #279 separately.