Add hex_tarball:unpack_docs/1

[wojtekmach]

Validations:

• the same size validations as we currently do for package tarballs
• ensure top-level file is not a valid semver

[wojtekmach]

We've introduced the function in #74, all that is left is the semver validation.

[wojtekmach]

I guess we should have the same validation when creating docs tarball.