Add border radius for wiki pages (#29937)

Before

<img width="1312" alt="image"
src="https://github.com/go-gitea/gitea/assets/81045/26a6dec2-9fea-4c0c-b6fb-290eab12a55a">

After

<img width="1298" alt="image"
src="https://github.com/go-gitea/gitea/assets/81045/01f7a714-eae9-4729-918f-3b4795094d0b">

web_src/css/repo/wiki.css

@@ -21,6 +21,7 @@
 
 .repository.wiki .wiki-content-parts .markup {
   border: 1px solid var(--color-secondary);
+  border-radius: var(--border-radius);
   padding: 1em;
   margin-top: 1em;
   font-size: 1em;