Use meta variables for all doc URLs to avoid obsolete links

[yebai]

Let us create a Quarto meta variable for each doc page in a similar way to:

https://github.com/TuringLang/docs/blob/20209fe4bed0b6fffc7b9c3d8941dd4ce9fb0d6d/_quarto.yml#L163-L164

Usage example: https://github.com/TuringLang/docs/blob/20209fe4bed0b6fffc7b9c3d8941dd4ce9fb0d6d/404.qmd#L7

to avoid the fragility of relative URLs when renaming and moving doc pages.

[beingPro007]

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