fix: stop amputating RC suffixes from docs URLs (#23903)

Fixes #23897 (docs link only — naming rename is in #23905)

- Fix version stripping logic in both Go (`codersdk/deployment.go`) and
TypeScript (`site/src/utils/docs.ts`) to preserve `-rc.X` suffixes
instead of amputating them along with `-devel`
- Add `v0.0.0` fallback in the TS frontend to match Go backend behavior
for dev builds
- Add tests covering RC, devel, and plain release version strings

> 🤖 Written by a Coder Agent. Will be reviewed by a human.

codersdk/deployment.go

@@ -1251,7 +1251,11 @@ func DefaultSupportLinks(docsURL string) []LinkConfig {
 }
 
 func removeTrailingVersionInfo(v string) string {
-	return strings.Split(strings.Split(v, "-")[0], "+")[0]
+	// Strip build metadata (everything after '+').
+	v, _, _ = strings.Cut(v, "+")
+	// Strip '-devel' suffix if present.
+	v = strings.TrimSuffix(v, "-devel")
+	return v
 }
 
 func DefaultDocsURL() string {