Add Basic API option for MongoDB

[klirix]

• Adds examples of Basic session API for MongoDB
• Lists it on the pages listing examples

With some differences in data structuring in SQL vs NoSQL, I embedded the session documents into user documents. This way we only need one operation per update and one operation for fetching both user and the session

Ideally the user and session object wouldn't be separated, as in DB they exist embedded together, but in order to conform to API's down the line that would rely on separate session and user object I splitted them up

And since the userId field in session is not used in application code, but only exists in the DB for joins, I ommited that field for unnecessary redundancy

I used sqlite as a base doc. Should be fine