Add SEP superseded state

Author: Joffref

docs/community/sep-guidelines.mdx

@@ -53,11 +53,12 @@ SEPs can be one one of the following states
 
 - `proposal:` SEP proposal without a sponsor.
 - `draft` : SEP proposal with a sponsor.
-- `in-review`: SEP proposal ready for review
-- `accepted`: SEP accepted by core maintainers, but still requires final wording and reference implementation
-- `rejected`: SEP rejected by core maintainers
+- `in-review`: SEP proposal ready for review.
+- `accepted`: SEP accepted by core maintainers, but still requires final wording and reference implementation.
+- `rejected`: SEP rejected by core maintainers.
 - `withdrawn`: SEP withdrawn.
-- `final`: SEP finalized
+- `final`: SEP finalized.
+- `superseded`: SEP has been replaced by a newer SEP.
 
 ### SEP Review & Resolution