Changes in the user guide layout

[luisaFelixSalles]

This PR changes the user guide section

• Add the tutorials subsection
• Adapt the index page of the main page and of the subsections (all the cards design)
• Change the sidebar: add captions for each subsection

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## doc/general-layout #1763 +/- ## ===================================================== Coverage ? 87.10% ===================================================== Files ? 83 Lines ? 9948 Branches ? 0 ===================================================== Hits ? 8665 Misses ? 1283 Partials ? 0 ```

[PProfizi]

@luisaFelixSalles the doc step is failing because the create_sites_for_python_operators.* scripts should not be moved.

[luisaFelixSalles]

@luisaFelixSalles the doc step is failing because the create_sites_for_python_operators.* scripts should not be moved.

You prefer if I change the reference in the other file or if I replace it ? @PProfizi