Fix loading of assets in Blubber documentation on doc.wikimedia.org
Vitepress attempts to load assets (both JS and CSS) from doc.wikimedia.org/assets, instead of doc.wikimedia.org/releng/blubber/assets. Setting the base
option in Vitepress configuration should fix this problem.
Solves !74 (comment 74158). FYI @dancy
Bug: T352262