Robert Hensing 4a1e3ad48b doc: Render verbatim @docroot@ on contributing page ...

In rendered form:

```diff
-Add references to the manual using ..
+Add references to the manual using [links like this](@docroot@/example.md)
```

(cherry picked from commit 147a34c573)

2025-07-30 11:56:00 +00:00

..

manual

doc: Render verbatim @docroot@ on contributing page

2025-07-30 11:56:00 +00:00