[skip ci]Add basic_types.md document for segcore (#8116)

Signed-off-by: xiaofan-luan <xiaofan.luan@zilliz.com>

docs/design_docs/segcore/basic_types.md

@@ -0,0 +1,16 @@
+# Glossary 
+
+- `Collection`: Data table, contains multiple Segments
+- `Segment`: The memory structure for storing a piece of data supports concurrent insertion, deletion, query, index loading, monitoring and statistics
+- `Schema`: Definition of collection data format, including
+    - `vector<FieldMeta>`, Order list of FieldMeta 
+    - `isAutoId`. if set to True , default primary field is `RowId` and it is auto generated 
+    - `primaryKey` (when `isAutoId = False`), specify primary key field
+- `FieldMeta`: field properties, including
+    - `DataType` data type, including Int8...Int64, Float, Double, FloatVector, BinaryVector and String later
+    - `Dim` (when dataType is vector type): vector dimension
+    - `metric_type` (when dataType is vector type, optional): (when datatype is vector type, optional): the metrictype corresponding to this vector is related to the small batch index and can be empty
+    - `FieldName`: column name
+    - `FieldId`: unique number of the column
+    - (hidden) `FieldOffset`, which is the subscript of `vector<Field>` in the schema. The internal calculation of segcore is basically based on fieldoffset
+- `Span` is similar to STD:: span， It supports vector type data and can be implicitly converted to `SpanBase` for interface overwrite