Make the generate target also manage the docs

[elliottt]

Documentation should be added to the gh-pages branch when the generate target is run, though at the moment only extensions get moved over.

[elliottt]

• generate-docs will generate and commit the docs to the gh-pages branch
• generate-exts will generate and commit the extensions to the gh-pages branch
• deploy will push any changes on the gh-pages branch to the origin