DOCS/ENHANCEMENT: Scroll to top button in documentation
I've been using pandas for a while and am just now starting to contribute. One of the things that has bugged me for a while is that in the docs there is no scroll to top button. Some of the pages get pretty long, and I'm usually without a Home button on my laptop, so I end up _stuck_ (exaggeration) deep in the docs. 

I looked into templating with Sphinx and discovered that it's pretty easy to add this feature to all pages. Under pandas/doc/_templates, I created a file layout.html (see code below) that adds a small bit of HTML, CSS, and JS which create a scroll to top button. I kept it to a basic implementation, but feel free if anyone has any suggestions about style or the breakpoint for viewing the button. 

**pandas/doc/_templates/layout.html**

```
{% extends "!layout.html" %} {%- block footer %}
<style type="text/css">
    .scrollToTop {
        text-align: center;
        font-weight: bold;
        position: fixed;
        bottom: 60px;
        right: 40px;
        display: none;
    }
</style>
<a href="#" class="scrollToTop">Scroll To Top</a>
<script type="text/javascript">
$(document).ready(function() {
    //Check to see if the window is top if not then display button
    $(window).scroll(function() {
        if ($(this).scrollTop() > 200) {
            $('.scrollToTop').fadeIn();
        } else {
            $('.scrollToTop').fadeOut();
        }
    });

    //Click event to scroll to top
    $('.scrollToTop').click(function() {
        $('html, body').animate({
            scrollTop: 0
        }, 500);
        return false;
    });
});
</script>
{% endblock %}
```
