DOC Move helpers section to dev developer guide (#1856)

It was in the "Adapters" section, which doesn't really fit.

docs/source/_toctree.yml

@@ -45,6 +45,8 @@
  title: Troubleshooting
  - local: developer_guides/checkpoint
  title: PEFT checkpoint format
+ - local: package_reference/helpers
+ title: Helpers
 
 - title: 🤗 Accelerate integrations
  sections:
@@ -110,8 +112,6 @@
  title: Layernorm tuning
  - local: package_reference/vera
  title: VeRA
- - local: package_reference/helpers
- title: Helpers
  title: Adapters
  - sections:
  - local: package_reference/merge_utils