[ZEPPELIN-2036] add documentation on separating workspaces (public/private)
[ZEPPELIN-2036] add documentation on separating workspaces (public/private)

### What is this PR for?
This is to add more description in documentation about notebook workspaces. Also some details from user mailing list [here](https://lists.apache.org/thread.html/d94276521942c90cca1325514ea93b737a517679dd0f6f7eb287492a%3Cusers.zeppelin.apache.org%3E)

### What type of PR is it?
Improvement | Documentation

### Todos
* [x] - add desc

### What is the Jira issue?
[ZEPPELIN-2036](https://issues.apache.org/jira/browse/ZEPPELIN-2036)

### How should this be tested?
green CI/ if description clear

### Screenshots (if appropriate)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: Khalid Huseynov <khalidhnv@gmail.com>

Closes #1965 from khalidhuseynov/docs/private-workspace and squashes the following commits:

fae6410 [Khalid Huseynov] fix formating: add lines
c85ee6e [Khalid Huseynov] rewording
cec8bbd [Khalid Huseynov] add public vs. private
8703770 [Khalid Huseynov] add description on separating workspaces

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