Add local area documentation

Author: vahid-ahmadi

docs/_toc.yml

@@ -2,11 +2,11 @@ format: jb-book
 root: intro
 chapters:
 - file: methodology.ipynb
+- file: constituency_methodology.ipynb
+- file: LA_methodology.ipynb
 - file: validation/index.md
   sections:
   - file: validation/national.ipynb
   - file: validation/constituencies.ipynb
   - file: validation/local_authorities.ipynb
-- file: pension_contributions.ipynb
-- file: constituency_methodology.ipynb
-- file: LA_methodology.ipynb
+- file: pension_contributions.ipynb