[SPARK-28792][SQL][DOC] Document CREATE DATABASE statement in SQL Reference
[SPARK-28792][SQL][DOC] Document CREATE DATABASE statement in SQL Reference

### What changes were proposed in this pull request?
Document CREATE DATABASE statement in SQL Reference Guide.

### Why are the changes needed?
Currently Spark lacks documentation on the supported SQL constructs causing
confusion among users who sometimes have to look at the code to understand the
usage. This is aimed at addressing this issue.

### Does this PR introduce any user-facing change?
Yes.

### Before:
There was no documentation for this.
### After:
![image](https://user-images.githubusercontent.com/29914590/65037831-290e2900-d96c-11e9-8563-92e5379c3ad1.png)
![image](https://user-images.githubusercontent.com/29914590/64858915-55f9cd80-d646-11e9-91a9-16c52b1daa56.png)

### How was this patch tested?
Manual Review and Tested using jykyll build --serve

Closes #25595 from sharangk/createDbDoc.

Lead-authored-by: sharangk <68bcc90f7c2c67f2f5d041a9a138bb7c6e9c06ad@gmail.com>
Co-authored-by: Xiao Li <905db6c57c9ba2de5402239d136139da9b5d0058@gmail.com>
Signed-off-by: Xiao Li <905db6c57c9ba2de5402239d136139da9b5d0058@gmail.com>

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