Fix#31395
This regression is introduced by #30273. To find out how GitHub handles
this case, I did [some
tests](https://github.com/go-gitea/gitea/issues/31395#issuecomment-2278929115).
I use redirect in this PR instead of checking if the corresponding `.md`
file exists when rendering the link because GitHub also uses redirect.
With this PR, there is no need to resolve the raw wiki link when
rendering a wiki page. If a wiki link points to a raw file, access will
be redirected to the raw link.
