docs(registry-operator): selected static site generator for documentation

[shanduur]

Proposed Architectural Decision Record (mADR)

Checklist

• [x] The context and problem statement are clearly articulated.
• [x] The considered options are listed with relevant links or references.
• [x] The chosen option is clearly stated along with the rationale.
• [x] The mADR template is filled out accurately.
• [x] Any additional information or references are included.

Closes #5