Show we have a list of "guides" inside Livebook itself

[hugobarauna]

I have seen people trying to find docs/guides about Livebook multiple times without knowing there's a Learn section inside Livebook that could answer some of their questions.

Although we plan to expand our docs at https://hexdocs.pm/livebook, in the meantime, maybe we can just add another entry to the main menu with a "Guides" title, something like that:

In that section, we could explain multiple guides inside the Livebook itself.

This would help people who go to the Livebook website looking for docs.