Docs redesign in line with upcoming website revamp

[fhennig]

We want to keep our design synchronized with the main stackable page. This means we should update these pages:

• https://docs.stackable.tech/home/stable/index.html (the docs)
• https://crds.stackable.tech/ (the CRD docs)

Changes should be mostly colors and fonts, nothing structural.

• Make the header and footer white
• Change the font of headings if possible - https://github.com/stackabletech/documentation-ui/pull/71

[fhennig]

For the documentation: https://github.com/stackabletech/documentation-ui/pull/73 - Preview PR

[fhennig]

for the CRD docs: https://github.com/stackabletech/crddocs/issues/39 - https://github.com/stackabletech/crddocs/pull/40