Add `mutual_information` and `classical_mutual_information` to `quantum_info.entropies`

[renatomello]

Checklist:

• [x] Reviewers confirm new code works as expected.
• [x] Tests are passing.
• [x] Coverage does not decrease.
• [x] Documentation is updated.

[codecov[bot]]

Codecov Report

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

Project coverage is 99.94%. Comparing base (df709ce) to head (3ba442d). Report is 21 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1455 +/- ## ======================================= Coverage 99.94% 99.94% ======================================= Files 81 81 Lines 11708 11718 +10 ======================================= + Hits 11701 11711 +10 Misses 7 7 ``` | [Flag](https://app.codecov.io/gh/qiboteam/qibo/pull/1455/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/qiboteam/qibo/pull/1455/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam) | `99.94% <100.00%> (+<0.01%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=qiboteam#carryforward-flags-in-the-pull-request-comment) to find out more.

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