Security Manager and Sandbox documentation

[alisevych]

Description

Documentation on Security Manager and Sandbox use cases.

Details

How Java Security Manager is used by UTBot.
How user code is sandboxed and corresponding tests are disabled - for safety reasons.
How a user can customize this, add some permissions or explicitly set useSandbox to true/false in settings.properties.

Context

Testing notes on the subject with links to the main issue and PR - there are some more details