Convert Trainer doc page to MarkDown (#14753)

* Convert Trainer doc page to MarkDown

* Fix repo consistency

* Fix the doc build test job

.github/workflows/build_doc_test.yml

@@ -46,4 +46,4 @@ jobs:
 
       - name: Make documentation
         run: |
-          doc-builder build transformers ./transformers/docs/source
+          doc-builder build transformers ./docs/source

utils/check_repo.py

@@ -507,6 +507,8 @@ DEPRECATED_OBJECTS = [
     "xnli_output_modes",
     "xnli_processors",
     "xnli_tasks_num_labels",
+    "TFTrainer",
+    "TFTrainingArguments",
 ]
 
 # Exceptionally, some objects should not be documented after all rules passed.