Add version v28 (#564)

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Reviewed-on: https://gitea.com/gitea/docs/pulls/564
Reviewed-by: bircni <[email protected]>
Co-authored-by: Lunny Xiao <[email protected]>

cloudflare/_redirects

@@ -9,9 +9,9 @@
 
 # The version served at the root of a product is also reachable under its
 # number, which is what the release notes and old links use.
-/api/1.27/* /api/:splat 301
+/api/28/* /api/:splat 301
 /runner/3/* /runner/:splat 301
-/1.27/* /:splat 301
+/28/* /:splat 301
 
 # The local search plugin had a results page of its own, algolia docsearch is a
 # modal opened from any page.