Add "Fork us" button to home webpage

Right now `gh-pages` branch has the web documentation of `scikit-api`. It's better if we can add a "Fork Us" button at the top of the web page as most of the documentation websites do these days.

That button will redirect the user to a new tab with this repository on github

Start by checking the gh-pages branch and locate the home page for the web documentation. Add a visible "Fork Us" button at the top, linking to this GitHub repository and opening it in a new tab; verify the rendered page and link behavior.

Written by the indexing model from the issue text.