docs, cmd: fix broken docs source links regression in vdoc (#23889)

Author: larpon

cmd/tools/vdoc/vdoc.v

@@ -303,12 +303,18 @@ fn (mut vd VDoc) generate_docs_from_file() {
 	} else {
 		os.dir(cfg.input_path)
 	}
-	manifest_path := os.join_path(dir_path, 'v.mod')
+	manifest_path := if cfg.is_vlib {
+		os.join_path(vroot, 'v.mod')
+	} else {
+		os.join_path(dir_path, 'v.mod')
+	}
 	if os.exists(manifest_path) {
 		vd.vprintln('Reading v.mod info from ${manifest_path}')
 		if manifest := vmod.from_file(manifest_path) {
 			vd.manifest = manifest
 		}
+	} else if cfg.is_vlib {
+		assert false, 'vdoc: manifest does not exist for vlib'
 	}
 	if cfg.include_readme || cfg.is_vlib {
 		mut readme_name := 'README'