Back and next in blog post should honored webRootPrefix

[Ducasse]

Right now we get

in https://pillar-markup.github.io/FoliageSample/news/2025-05-09-Moon.html

https://pillar-markup.github.io/news/2025-05-18-Balls.html while it should be https://pillar-markup.github.io/FoliageSample/news/2025-05-18-Balls.html

[Ducasse]

I fixed this as shown in Foliage Sample but I do not see why this is not working for PharoWeb20.