[SPARK-31465][SQL][DOCS] Document Literal in SQL Reference

### What changes were proposed in this pull request?
Document Literal in SQL Reference

### Why are the changes needed?
Make SQL Reference complete

### Does this PR introduce any user-facing change?
Yes
<img width="1049" alt="Screen Shot 2020-04-22 at 8 50 04 PM" src="https://user-images.githubusercontent.com/13592258/80057912-9ecb0c00-84dc-11ea-881e-1415108d674f.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 50 29 PM" src="https://user-images.githubusercontent.com/13592258/80057917-a12d6600-84dc-11ea-8884-81f2a94644d5.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 50 54 PM" src="https://user-images.githubusercontent.com/13592258/80057922-a4c0ed00-84dc-11ea-9857-75db50f7b054.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 51 15 PM" src="https://user-images.githubusercontent.com/13592258/80057927-a7234700-84dc-11ea-9124-45ae1f6143fd.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 51 44 PM" src="https://user-images.githubusercontent.com/13592258/80057932-ab4f6480-84dc-11ea-8393-cf005af13ce9.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 52 03 PM" src="https://user-images.githubusercontent.com/13592258/80057936-ad192800-84dc-11ea-8d78-9f071a82f1df.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 52 28 PM" src="https://user-images.githubusercontent.com/13592258/80057940-b0141880-84dc-11ea-97a7-f787cad0ee03.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 53 14 PM" src="https://user-images.githubusercontent.com/13592258/80057945-b30f0900-84dc-11ea-985f-c070609e2329.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 53 34 PM" src="https://user-images.githubusercontent.com/13592258/80057949-b5716300-84dc-11ea-9452-3f51137fe03d.png">

<img width="1050" alt="Screen Shot 2020-04-22 at 8 53 56 PM" src="https://user-images.githubusercontent.com/13592258/80057957-b904ea00-84dc-11ea-8b12-a6f00362aa55.png">

<img width="1049" alt="Screen Shot 2020-04-22 at 8 54 12 PM" src="https://user-images.githubusercontent.com/13592258/80057962-bacead80-84dc-11ea-94da-916b1d1c1756.png">

### How was this patch tested?
Manually build and check

Closes #28237 from huaxingao/literal.

Authored-by: Huaxin Gao <huaxing@us.ibm.com>
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>
(cherry picked from commit 03fe9ee)
Signed-off-by: Takeshi Yamamuro <yamamuro@apache.org>

docs/_data/menu-sql.yaml

@@ -78,6 +78,8 @@
   subitems:
     - text: Data Types
       url: sql-ref-datatypes.html
+    - text: Literals
+      url: sql-ref-literals.html
     - text: Null Semantics
       url: sql-ref-null-semantics.html
     - text: NaN Semantics