Add a new link to User Guide

[josecelano]

I've moved the documentation for end users to this repo:

https://github.com/torrust/torrust-index-gui-user-guide

It's deployed to GitHub pages here:

https://torrust.github.io/torrust-index-gui-user-guide/

For the time being, it is elementary, but we are planning to add more sections.

It would be helpful to add a new "Help" link to this menu:

So users can easily find the documentation.

[anujkumardarji]

I have created pull request #548 to address this issue. Please review and let me know if any further changes are needed.