Feature/hero-image gradient parameter

[law]

This adds a boolean site-parameter to control the application of the bg-gradient-to-r darkening css on a Hero image. For backwards compatibility/'principle-of-least-astonishment', this parameter is set to 'true' by default, maintaining the previous default behavior of darkening the hero image via CSS.

This also applies the Tailwind CSS 'drop-shadow-md' property to the Author and Site Title sections. This will be invisible over a dark image, but aids in readability over a light image (such as what may get displayed if homepageImageGradientOverlay is set to false).

Please don't hesitate to suggest changes if there is a cleaner/better/less-hateful way to do this - I am not the strongest when it comes to FE HTML/CSS stuff, but I am always eager to learn and improve.

[nunocoracao]

Hello @law thanks for the PR. I am sorry but I'll be closing it whatever. Please open a ticket so that we can discuss if this is the best implementation. E.g. this could be a different type of background. Also, we would need to add the relevant documentation so that people can use it.