Auto merge of #4329 - behnam:dev, r=matklad

[doc/manifest] Add build-dependencies to Dependency sections list

The `[build-dependencies]` section was missing from the Dependency
sections list, making it hard to find its existence and link to
`specifying-dependencies` page. This puts the section title in the list,
so a normal find works.

Background: https://github.com/rust-lang/cargo/issues/4309#issuecomment-317965620