Generation of documentation for the new libraries

Created by: ticki

Problem: The generation of documentation for the new programs and libraries is not automated.

Solution: Add it to make doc effectively making the documentation for the new crates be generated and uploaded to redox-os.org.

Changes introduced by this pull request:

• Add libmalloc submodule.
• Add doc-gen for libextra.
• Add doc-gen for libmalloc.
• Add doc-gen for binutils.
• Add doc-gen for zfs.
• Add doc-gen for sodium.

Drawbacks: Possibly slower doc generation.

TODOs:

• Add links to the website.
• Make private items appear too in the documentation.
• Add doc generation for the rest of the crates.

State: Ready.

Blocking/related: Part of the documentation effort, #525 (closed).