git-core: move docs to new port

[ryandesign (Ryan Carsten Schmidt)]

In the interest of having fewer variants and making it easier to add/remove documentation to/from one's git installation, I propose that the +doc variant be deleted from the git-core port and that a new port git-htmldocs be created. This name is based on the fact that the docs are distributed in a distfile whose name begins with "git-htmldocs". Or the port could be called "git-docs" (this would match enlightment-docs, gnucash-docs et al) or "git-doc" (this would match postgresql83-doc, python26-doc etc).