Document hugo extended requirement for custom CSS

[jpursey]

If the extended Hugo build is not used, then attempting to add a custom CSS file via the instructions (https://github.com/theNewDynamic/gohugo-theme-ananke#custom-css) will silently fail to work. This is due to changes in Hugo which by default automatically falls back on the theme's minified CSS if it can't build a new one.

It would be great if this requirement was called out in the documentation!

See #452 for the associated discussion thread.

[regisphilibert]

I'll fix the documentation for now, here's a poll to determine future action: #452 (comment)

[davidsneighbour]

See #719 and #718. This will come naturally with some updates we need to do. I will also add a mutable notice in CLI that the used version of Hugo does or does not support certain features.