Improve backend docstrings

[YannCabanes]

This PR improves the documentation about the backends to explain how they work.

We will add information about the backends (how it works, how to select the backend, what happens if it is not selected...) in the Quick-start guide: https://github.com/tslearn-team/tslearn/blob/main/docs/quickstart.rst and add a file named backend.rst in the folder main/docs.

We will also add information about the backends and examples in the docstrings of the functions dtw (https://github.com/tslearn-team/tslearn/blob/main/tslearn/metrics/dtw_variants.py) and soft_dtw (https://github.com/tslearn-team/tslearn/blob/main/tslearn/metrics/softdtw_variants.py).

[codecov-commenter]

Codecov Report

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

Comparison is base (09441ab) 93.33% compared to head (602268c) 93.64%.

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```diff @@ Coverage Diff @@ ## main #482 +/- ## ========================================== + Coverage 93.33% 93.64% +0.31% ========================================== Files 67 67 Lines 5728 5728 ========================================== + Hits 5346 5364 +18 + Misses 382 364 -18 ```

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[YannCabanes]

Hello @rtavenar, could you review this PR please?

[YannCabanes]

Pytest documentation to skip doctests when torch is not installed for the linux_without_torch test: https://docs.pytest.org/en/latest/how-to/usage.html# https://stackoverflow.com/questions/36456920/specify-which-pytest-tests-to-run-from-a-file/61869181#61869181 https://stackoverflow.com/questions/38442897/how-do-i-disable-a-test-using-pytest