Un-blacklist the layout crate from rustdoc. See #2924.

Whatever rustdoc bug we used to hit was solved by the Rust upgrade.
(The style crate still causes rustdoc to overflow its stack.)

Also use Markdown tables rather than ASCII diagrams in inlines.rs.
Not only does this look nicer, this works around rustdoc trying
(and failing) to parse them as Rust code.
Move documentation table stylining to a shared location.
Blacklisted docs don’t need any dependency, since we don’t actually generate them.
Merge pull request #3002 from servo/rustdoc-layout

Un-blacklist the layout crate from rustdoc.