Add more content into rustdoc book

[GuillaumeGomez]

https://doc.rust-lang.org/rustdoc/what-is-rustdoc.html is a very nice introduction, but maybe it's time to go further and provide maybe guidelines, more tips and customization chapters?

[AndyGauge]

I'm going to begin working on this. Many questions to follow :)

[AndyGauge]

Is this the kind of info we want to see:

• Add API documentation front page styleguide rfcs#1687
• https://rust-lang.github.io/api-guidelines/documentation.html

Also, I'd love to get an idea what customization can be done, haven't seen that in the wild.

Here's a list of customizations I've found:

• #[doc(alias="another")]

[AndyGauge]

Got started today on this. Here's a bit of the work:
AndyGauge#1
Is it in the right direction?