docs: Move developer docs from top-level to its own directory. (#1642)

docs/backend-multipart.md

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+## Multipart
+
+Each incoming part is uploaded in the following format.
+
+Placeholder uploads.json to indicate a leaf.
+
+ ```EXPORT_DIR/.minio/multipart/BUCKET/PATH/TO/OBJECT/uploads.json```
+
+Incomplete file
+
+ ```EXPORT_DIR/.minio/multipart/BUCKET/PATH/TO/OBJECT/UPLOADID/00000.incomplete```
+
+Actual parts
+
+ ```EXPORT_DIR/.minio/multipart/BUCKET/PATH/TO/OBJECT/UPLOADID/PART_NUMBER.MD5SUM_STRING```
+
+## FS Format.
+
+Each of these parts are concatenated back to a single contigous file.
+
+```EXPORT_DIR/BUCKET/PATH/TO/OBJECT```
+
+## XL Format.
+
+Each of these parts are kept as is in the following format.
+
+Special json file indicate the metata multipart information, essentially list of parts etc.
+
+```EXPORT_DIR/BUCKET/PATH/TO/OBJECT/00000.minio.multipart```
+
+All the parts that were uploaded.
+
+```EXPORT_DIR/BUCKET/PATH/TO/OBJECT/PART_NUMBER.minio.multipart```