Move Rust By Example to the Main Website

[projektir]

This is an issue for moving Rust By Example, which is currently at https://rustbyexample.com/, to https://doc.rust-lang.org/rust-by-example. I will be working on the first step shortly.

• Convert RBE to a submodule under rust-lang/rust/src/doc
• Add RBE to the bookshelf (thanks @QuietMisdreavus!)
• Update book release process on forge

After it is converted and RBE reaches stable:

• Update the main English website link
• Make https://rustbyexample.com/ a 301

[projektir]

Step one done! Now waiting for this to reach stable.