Prevent jekyll version mixup in docs

The new docs do not publish to getbootstrap.dev:9001, they publish to localhost:9001. The "Customize and Download" button was fixed to handle this in [8158](https://github.com/twitter/bootstrap/pull/8158) but the docs were not updated.

There have been a number of comments (e.g. [8174](https://github.com/twitter/bootstrap/issues/8174)) where people are trying to use the new Jekyll 1.x syntax with the old version of the library. This updates the docs to explain that the instructions are for 1.x as well as changing the link to point to localhost