Implement docs site redesign and add dark mode

What does this MR do and why?

Implements the Docs site redesign. ✨ This branch/MR consists of several other MRs which have all been reviewed/merged individually.

See Docs site "look and feel" updates (&3) • Suzanne Selhorn, Sarah German+ • Next 1-3 releases for details and screenshots.

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.