fix(docs): Replace github tree links with permalinks

If the links just point to the tree, the content they
point to might change over time. These links don't need
the content to be up-to-date. Compared to where they
pointed to in the Unikraft source tree when these links
were initially created, they still point to the right stuff.

Signed-off-by: Thassilo Schulze <thassilo@unikraft.io>

content/docs/contributing/coding-conventions.mdx

@@ -13,8 +13,8 @@ Maintainers would be the ones who judge if there are certain corner cases when a
 Some example of extreme cases:
 
 * Having line with more than 80 characters, as they are easily searchable.
-  You can see an example [here](https://github.com/unikraft/unikraft/blob/staging/plat/drivers/virtio/virtio_pci.c#L141C1-L141C87).
-* Using `typedef struct` to enhance readability, as you can see [here](https://github.com/unikraft/unikraft/blob/staging/plat/kvm/include/kvm-x86/multiboot.h#L96).
+  You can see an example [here](https://github.com/unikraft/unikraft/blob/325daa416e55df74824d90f9df9470433b8da316/plat/drivers/virtio/virtio_pci.c#L141).
+* Using `typedef struct` to enhance readability, as you can see [here](https://github.com/unikraft/unikraft/blob/325daa416e55df74824d90f9df9470433b8da316/plat/kvm/include/kvm-x86/multiboot.h#L96).
 
 ## Basics