Starting Minimal Flex + Section Options + Reduce same bg color padding

[yuvilio]

We've been re-using a pattern recently of having built in ways to select background colors (and sometimes images) to sections. That included figuring out to smartly prevent double padding (Bottom padding of a section + top padding of it's following one) when both have the same background color. That way those don't look too distant from one another.

• [x] Let's add some minimal Flex Sections starting json, (the main Flex section and Shared: Section Options at the minimum ]. To add some consistency with bubs-next, we can re-use the field groups from there.
• [x] Let's tweak the flex template twig to use these section options (slug, background color, etc) , including the twig logic to prevent double padding

[yuvilio]

These adjusts are now in per PR #173. See sample screenshot there for demonstration.