Added Gentoo live ebuild

Still needed to push this ebuild to some overlay or main tree; otherwise it requires more effort to use.

Note: this does not use Makefile found in docs/ directory using sphinx directly (though archlinux PKGBUILD seems to ignore it anyway, with the docs themselves).

Can you tell what for there is docutils dependency in `setup.py`? I see no direct references, and sphinx already depends on docutils. I have added this dependency to the ebuild, but it should probably be removed.