new: start harvesting pipeline documentation

- Old vs new general pipeline structure

harvesting.md

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+# Harvesting Pipeline Structure
+
+## Old Harvesting Logic
+Unique to each file + schema format
+```mermaid
+flowchart LR
+  sc([SOURCE CREATION])
+  gs([GATHER STAGE])
+  fs([FETCH STAGE])
+  is([IMPORT STAGE])
+  sc --> gs
+  gs --> fs
+  fs --> is
+```
+
+## New Harvesting Logic
+Universal to all file + schema formats
+```mermaid
+flowchart TD
+  sc([SOURCE CREATION])
+  extract([Extract Catalog Source])
+  compare([Compare Source Catalog to Data.gov Catalog])
+  nochanges{No Changes?}
+  deletions{Datasets to Delete?}
+  updates{Datasets to Add or Update?}
+  load([Load into Data.gov Catalog])
+  validate([Validate Dataset])
+  transform([Transform Schema of Dataset])
+  completed([End])
+
+  sc --> extract
+  extract --> compare
+  compare --> deletions
+  compare --> updates
+  deletions --> load
+  updates --> validate
+  validate --> transform
+  transform --> validate
+  validate --> load
+  load --> completed
+  compare --> nochanges
+  nochanges --> completed
+```