[skip ci] A doc to explain hybrid timestamp (TSO) (#12002)

Signed-off-by: dragondriver <jiquan.long@zilliz.com>

docs/design_docs/milvus_hybrid_ts_en.md

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+# Hybrid Timestamp in Milvus
+
+In chapter [Milvus TimeSync Mechanism](./milvus_timesync_en.md), we have already known why we need TSO in Milvus. Milvus
+borrows the [TiKV's](https://github.com/tikv/tikv) implementation into TSO. So if you are interested in how TSO was
+implemented, you can look into the official documentation of TiKV.
+
+This chapter will only introduce two points:
+
+- 1, the organization of hybrid TSO in Milvus;
+- 2, how should we parse the hybrid TSO;
+
+## The Organization of TSO
+
+The type of TSO is `uint64`. As shown in the figure below, TSO was organized by two parts:
+
+- 1, physical part;
+- 2, logical part;
+
+The front 46 bits is of physical part, and the last 18 bits is of logical part.
+
+Note, physical part is the UTC time in Milliseconds.
+
+![Timestamp struct](./graphs/time_stamp_struct.jpg)
+
+For some users such as DBAs, they will want to sort the operations and list them in UTC time order.
+
+Actually, we can use the TSO order to sort the `Insert` operations or `Delete` operations.
+
+So the question becomes how we get the UTC time from TSO.
+
+As we have described above, physical part consists the front 46 bits of TSO.
+
+So given a TSO which is returned by `Insert` or `Delete`, we can directly shift left 18 bits to get the UTC time.
+
+For example in Golang:
+
+```go
+const (
+	logicalBits     = 18
+	logicalBitsMask = (1 << logicalBits) - 1
+)
+
+// ParseTS parses the ts to (physical,logical).
+func ParseTS(ts uint64) (time.Time, uint64) {
+	logical := ts & logicalBitsMask
+	physical := ts >> logicalBits
+	physicalTime := time.Unix(int64(physical/1000), int64(physical)%1000*time.Millisecond.Nanoseconds())
+	return physicalTime, logical
+}
+```
+
+In Python:
+
+```python
+>>> import datetime
+>>> LOGICAL_BITS = 18
+>>> LOGICAL_BITS_MASK = (1 << LOGICAL_BITS) - 1
+>>> def parse_ts(ts):
+...     logical = ts & LOGICAL_BITS_MASK
+...     physical = ts >> LOGICAL_BITS
+...     return physical, logical
+... 
+>>> ts = 429164525386203142
+>>> utc_ts_in_milliseconds, _ = parse_ts(ts)
+>>> d = datetime.datetime.fromtimestamp(utc_ts_in_milliseconds / 1000.0)
+>>> d.strftime('%Y-%m-%d %H:%M:%S')
+'2021-11-17 15:05:41'
+>>> 
+```