Auto merge of #4453 - behnam:book, r=alexcrichton

Import mdBook-based docs and sync

Since we decided to keep the mdBook-based docs in-repo, I have imported the existing converted docs from <https://github.com/istankovic/cargo-book> under `/src/doc/book/` here, and have synced the current docs and the mdBook-based ones manually, file-by-file.

I have created a `MIGRATION_MAP` file, which shows the relationship between old docs and new ones. The first column is the old file, the second column is the canonical location in mdBook, and the rest of columns are globs of other files in mdBook containing content from the old file.

The first and second column of `MIGRATION_MAP` shall later be used to create redirect rules from `doc.crates.io/` to `doc.rust-lang.org/cargo/`.

I will also add a README file to remind everyone to keep these files in sync during the migration.

There are also two or three small wording fixes here, which I'll note inline.

This is a retry of <https://github.com/rust-lang/cargo/pull/4220>.

First step for <https://github.com/rust-lang/cargo/issues/4040>.