fix: export LogFormatter + update docs (#479)

Author: dreamorosi

Diff for: docs/core/logger.md

@@ -625,13 +625,13 @@ This is how the `MyCompanyLogFormatter` (dummy name) would look like:
 === "utils/formatters/MyCompanyLogFormatter.ts"
 
     ```typescript
-    import { LogFormatter } from "@aws-lambda-powertools/logger";
-    import { LogAttributes, UnformattedAttributes } from "@aws-lambda-powertools/logger/types";
+    import { LogFormatter } from '@aws-lambda-powertools/logger';
+    import { LogAttributes, UnformattedAttributes } from '@aws-lambda-powertools/logger/lib/types';
 
     // Replace this line with your own type
     type MyCompanyLog = LogAttributes;
 
-    class MyCompanyLogFormatter implements LogFormatter {
+    class MyCompanyLogFormatter extends LogFormatter {
 
         public formatAttributes(attributes: UnformattedAttributes): MyCompanyLog {
             return {

Diff for: packages/logger/src/index.ts

@@ -1,3 +1,4 @@
 export * from './helpers';
 export * from './Logger';
-export * from './middleware';
+export * from './middleware';
+export * from './formatter';