Add D2 support

[lextm]

Checklist

• I have read and accepted the contributing guidelines.

Is your feature request related to a problem? Please describe

D2 is an option much more extensive than Mermaid,

https://d2lang.com/

and has been widely adopted in the industry and other static site solutions.

Describe the solution you'd like

I integrated D2 with this theme in my own blog recently but wanted to raise a feature request here so maybe one day it becomes part of this theme by default.

Describe alternatives you've considered

Clearly people can follow my example to integrate directly at Jekyll level.

Additional context

No response