Create documentation

The hpc binary is currently documented in the GHC User Guide: https://downloads.haskell.org/ghc/latest/docs/users_guide/profiling.html#the-hpc-toolkit

As discussed in this comment: ghc/ghc#21870 (comment 443139) We should probably separate out the documentation and replace the link in the GHC user guide with a link to it.

In order to stay with the "corporate design" of Haskell tooling documentation, we could probably use Sphinx + readthedocs? The documentation should live in a docs subdirectory of this repository.