Add README and example do-file for dea_extensions
Add dea_extensions folder with 15 extension commands; update dea.sthlp DOIs
dea.sthlp: add DOIs to all references, update CPY1999/DPT1984 entries
docs: add README.md for v2.0
v2.0: remove legacy files from trunk root (archived in old/)
v2.0: unified dea.ado, move legacy files to old/
tag: archive legacy version before v2.0 unified rewrite
Revision & Enhanced comments and structure for clarity.