add service desk to comparison (#6220)

* add service desk to comparison

* add link to issue

Author: MartinDelille

docs/content/doc/features/comparison.en-us.md

@@ -88,6 +88,7 @@ _Symbols used in table:_
 | Issue search | ✓ | ✘ | ✓ | ✓ | ✓ | ✓ | ✘ |
 | Global issue search | ✘ | ✘ | ✓ | ✓ | ✓ | ✓ | ✘ |
 | Issue dependency | ✓ | ✘ | ✘ | ✘ | ✘ | ✘ | ✘ |
+| Service Desk | [✘](https://github.com/go-gitea/gitea/issues/6219) | ✘ | ✘ | ✘ | ✓ | ✘ | ✘ |
 
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