Make Fixed Sidebar in `keras.io` website!

[innat]

trafficstars

I found it very unpleasant when navigating the keras.io site. The sidebar should be fixed (something we see in the official tensorflow.org.).

Ref. Fixed Sidebar

[innat]

@fchollet

[innat]

I was referring something like as follows

https://jax.readthedocs.io/en/latest/index.html