feat: add parallax on hero

[sergiopena]

Add boolean to set the hero background image as fixed to create a parallax scroll effect.

What this PR does / why we need it:
Adds a nice parallax effect to the Hero partial

Which issue this PR fixes
partially fixes #814

Special notes for your reviewer:
I started learning Hugo and this theme seven hours ago, so I literally don't know what I'm doing.

Just saw that it looked super easy to add a bool to fix the image in the background and get the effect 😇

It works on my computer ™️

Release note:

 parallax effect to hero partial

[stp-ip]

Thank you for the addition already.
There are a few missing things.
It isn't documented on the fragment page: https://github.com/okkur/syna/tree/master/exampleSite/content/fragments/hero
Additionally with the default image it seems to not work well. Maybe a two part screenshot attached to the PR showcasing this effect makes this easier to grasp for anyone following up.