Add the information regarding ESMValTool tutorial in "Getting started" section of the documentation.

[egalytska]

This is the follow-up on the discussion on adding the information regarding the ESMValTool tutorial to the documentation (so it is visible and easy to spot). Together with @LisaBock , @gulcing and @rswamina we discussed that it would be great to add to the Getting started part a sentence saying:

"The tutorial is a good place to start. Check it out. https://tutorial.esmvaltool.org/".

In turn, in the Introduction part of the Tutorial, we will add a sentence saying "For a more detailed overview of the ESMValTool functionalities see ESMValTool documentation"

In this perspective, we also discussed whether it would be useful to switch the order of ESMValTool Functionalities and Getting started in the Documentation. Community feedback is very welcome.

[bouweandela]

@egalytska I opened #3628 last week to add something along those lines. Does that look alright or would you like a different text? If so, please comment on the pull request.