Make the Rust Book a mobile-friendly site

[Valloric]

Results from Google's Mobile-Friendly Test.

[huonw]

For future triage/comparison purposes, the current state is:

Reasons this page is not mobile-friendly

• Text too small to read
• Links too close together
• Mobile viewport not set

[Gankra]

@Valloric are you tackling this or just giving a heads up?

[Jojoshua]

It would also be nice if it was searchable.

[Valloric]

@gankro Giving a heads-up. :)

[kmcallister]

Folding this into #20850.