Add relevant titles to pages

[meven]

Fixes #330
I would welcome feedback if any.

It adds better title (as in the <title>) to pages.

Site path	page title
/	Rust programming language
/lean	Learn - Rust programming language
/learn/get-started	Get started - Rust programming language
/what/cli	Cli - Rust programming language
/what/wasm	Wasm - Rust programming language
/what/networking	Networking - Rust programming language
/what/embedded	Embedded - Rust programming language
/tools	Tools - Rust programming language
/community	Community - Rust programming language
/governance	Governance - Rust programming language
/governance/teams/moderation	Moderation team - Rust programming language
/governance/teams/crates-io	Crates.io team - Rust programming language
/governance/wgs/wasm	Wasm team - Rust programming language
/policies/code-of-conduct	Code of conduct - Rust programming language
/policies/licenses	Licenses - Rust programming language
/production	Production - Rust programming language
/production/users	Users - Rust programming language

I did not follow exactly #330 (comment)

I followed a pattern similar to the Rust book for instance https://doc.rust-lang.org/book/2018-edition/ch03-03-how-functions-work.html
In the browser tab we see the relevant part even when the tab has a narrow width but we still keep some context.

Capitalization could need a change, or a "The" before "Rust programming language" could be relevant.

I would welcome any feedback ;)

[meven]

Rust 2018 is already two days away ! 😀
I would love to fix the issue at hand that.

[chriskrycho]

My take is that this is good enough for the moment! We might want to use titlecase in a follow-on update, but at a minimum this gets us some good page titles for SEO and at-a-glance, which we need for the Edition release.