fix(v2): never remove trailing slash from site root like '/baseUrl/' (#5082)

* never apply trailingSlash to site root ('/baseUrl/') => only subroutes

* add deprecation comment for loadContext.baseUrl in favor of loadContext.siteConfig.baseUrl

* commit typo

* useless code

packages/docusaurus-utils-common/src/index.ts

@@ -6,4 +6,6 @@
  */
 
 export {default as applyTrailingSlash} from './applyTrailingSlash';
+export type {ApplyTrailingSlashParams} from './applyTrailingSlash';
+
 export {default as uniq} from './uniq';