Prepared user documentation for 4.0 release (Lithium)

As we are starting to add features to master, we need to start updating the
user documentation. This commit is needed before we can ask contributors to provide user documentation together with the code.

Note
------

If you run ``sbt clean make-site``, you'll see the following warnings in the output

```
[error] current-user-doc/faq/index.rst:44: WARNING: unknown document: ../gettingstarted/index
[error] current-user-doc/faq/index.rst:56: WARNING: unknown document: ../gettingstarted/index
[error] current-user-doc/faq/known-issues.rst:15: WARNING: unknown document: ../advancedsetup/index
```

They should be ignored, as they don't create any issue.